diff --git a/services/horizon/internal/actions_root.go b/services/horizon/internal/actions_root.go
index 6ddb043081..0ab53a9455 100644
--- a/services/horizon/internal/actions_root.go
+++ b/services/horizon/internal/actions_root.go
@@ -22,7 +22,7 @@ func (action *RootAction) JSON() {
 		ledger.CurrentState(),
 		action.App.horizonVersion,
 		action.App.coreVersion,
-		action.App.networkPassphrase,
+		action.App.config.NetworkPassphrase,
 		action.App.protocolVersion,
 		action.App.config.FriendbotURL,
 	)
diff --git a/services/horizon/internal/actions_root_test.go b/services/horizon/internal/actions_root_test.go
index 504b26929f..2c6be04644 100644
--- a/services/horizon/internal/actions_root_test.go
+++ b/services/horizon/internal/actions_root_test.go
@@ -4,8 +4,8 @@ import (
 	"encoding/json"
 	"testing"
 
-	"github.com/stellar/go/services/horizon/internal/test"
 	"github.com/stellar/go/protocols/horizon"
+	"github.com/stellar/go/services/horizon/internal/test"
 )
 
 func TestRootAction(t *testing.T) {
@@ -23,6 +23,7 @@ func TestRootAction(t *testing.T) {
 
 	ht.App.horizonVersion = "test-horizon"
 	ht.App.config.StellarCoreURL = server.URL
+	ht.App.config.NetworkPassphrase = "test"
 	ht.App.UpdateStellarCoreInfo()
 
 	w := ht.Get("/")
diff --git a/services/horizon/internal/app.go b/services/horizon/internal/app.go
index c813a0ad92..67c2d27f65 100644
--- a/services/horizon/internal/app.go
+++ b/services/horizon/internal/app.go
@@ -5,13 +5,13 @@ import (
 	"database/sql"
 	"fmt"
 	"net/http"
+	"os"
 	"runtime"
 	"sync"
 	"time"
 
 	"github.com/gomodule/redigo/redis"
 	"github.com/rcrowley/go-metrics"
-	"github.com/stellar/go/build"
 	"github.com/stellar/go/clients/stellarcore"
 	horizonContext "github.com/stellar/go/services/horizon/internal/context"
 	"github.com/stellar/go/services/horizon/internal/db2/core"
@@ -32,22 +32,21 @@ import (
 
 // App represents the root of the state of a horizon instance.
 type App struct {
-	config            Config
-	web               *Web
-	historyQ          *history.Q
-	coreQ             *core.Q
-	ctx               context.Context
-	cancel            func()
-	redis             *redis.Pool
-	coreVersion       string
-	horizonVersion    string
-	networkPassphrase string
-	protocolVersion   int32
-	submitter         *txsub.System
-	paths             paths.Finder
-	ingester          *ingest.System
-	reaper            *reap.System
-	ticks             *time.Ticker
+	config          Config
+	web             *Web
+	historyQ        *history.Q
+	coreQ           *core.Q
+	ctx             context.Context
+	cancel          func()
+	redis           *redis.Pool
+	coreVersion     string
+	horizonVersion  string
+	protocolVersion int32
+	submitter       *txsub.System
+	paths           paths.Finder
+	ingester        *ingest.System
+	reaper          *reap.System
+	ticks           *time.Ticker
 
 	// metrics
 	metrics                  metrics.Registry
@@ -64,7 +63,6 @@ func NewApp(config Config) (*App, error) {
 
 	result := &App{config: config}
 	result.horizonVersion = app.Version()
-	result.networkPassphrase = build.TestNetwork.Passphrase
 	result.ticks = time.NewTicker(1 * time.Second)
 	result.init()
 	return result, nil
@@ -270,8 +268,18 @@ func (a *App) UpdateStellarCoreInfo() {
 		return
 	}
 
+	// Check if NetworkPassphrase is different, if so exit Horizon as it can break the
+	// state of the application.
+	if resp.Info.Network != a.config.NetworkPassphrase {
+		log.Errorf(
+			"Network passphrase of stellar-core (%s) does not match Horizon configuration (%s). Exiting...",
+			resp.Info.Network,
+			a.config.NetworkPassphrase,
+		)
+		os.Exit(1)
+	}
+
 	a.coreVersion = resp.Info.Build
-	a.networkPassphrase = resp.Info.Network
 	a.protocolVersion = int32(resp.Info.ProtocolVersion)
 }
 
diff --git a/services/horizon/internal/config.go b/services/horizon/internal/config.go
index bbff804403..aefe06c6d4 100644
--- a/services/horizon/internal/config.go
+++ b/services/horizon/internal/config.go
@@ -21,6 +21,7 @@ type Config struct {
 	RateLimit              *throttled.RateQuota
 	RateLimitRedisKey      string
 	RedisURL               string
+	NetworkPassphrase      string
 	FriendbotURL           *url.URL
 	LogLevel               logrus.Level
 	LogFile                string
diff --git a/services/horizon/internal/helpers_test.go b/services/horizon/internal/helpers_test.go
index 245c915a19..5f6356aeb9 100644
--- a/services/horizon/internal/helpers_test.go
+++ b/services/horizon/internal/helpers_test.go
@@ -7,6 +7,7 @@ import (
 	"log"
 	"time"
 
+	"github.com/stellar/go/network"
 	"github.com/stellar/go/services/horizon/internal/test"
 	supportLog "github.com/stellar/go/support/log"
 	"github.com/throttled/throttled"
@@ -32,6 +33,7 @@ func NewTestConfig() Config {
 		},
 		ConnectionTimeout: 55 * time.Second, // Default
 		LogLevel:          supportLog.InfoLevel,
+		NetworkPassphrase: network.TestNetworkPassphrase,
 	}
 }
 
diff --git a/services/horizon/internal/init_ingester.go b/services/horizon/internal/init_ingester.go
index 7e6c0a16a6..2d33d987c1 100644
--- a/services/horizon/internal/init_ingester.go
+++ b/services/horizon/internal/init_ingester.go
@@ -11,12 +11,12 @@ func initIngester(app *App) {
 		return
 	}
 
-	if app.networkPassphrase == "" {
+	if app.config.NetworkPassphrase == "" {
 		log.Fatal("Cannot start ingestion without network passphrase.  Please confirm connectivity with stellar-core.")
 	}
 
 	app.ingester = ingest.New(
-		app.networkPassphrase,
+		app.config.NetworkPassphrase,
 		app.config.StellarCoreURL,
 		app.CoreSession(nil),
 		app.HorizonSession(nil),
diff --git a/services/horizon/internal/init_txsub.go b/services/horizon/internal/init_txsub.go
index 68963b0d61..cff314c56b 100644
--- a/services/horizon/internal/init_txsub.go
+++ b/services/horizon/internal/init_txsub.go
@@ -22,7 +22,7 @@ func initSubmissionSystem(app *App) {
 			History: &history.Q{Session: app.HorizonSession(nil)},
 		},
 		Sequences:         cq.SequenceProvider(),
-		NetworkPassphrase: app.networkPassphrase,
+		NetworkPassphrase: app.config.NetworkPassphrase,
 	}
 }
 
diff --git a/services/horizon/internal/txsub/open_submission_list.go b/services/horizon/internal/txsub/open_submission_list.go
index b38a5a6bab..43c066e942 100644
--- a/services/horizon/internal/txsub/open_submission_list.go
+++ b/services/horizon/internal/txsub/open_submission_list.go
@@ -29,7 +29,7 @@ type openSubmission struct {
 
 type submissionList struct {
 	sync.Mutex
-	submissions map[string]*openSubmission
+	submissions map[string]*openSubmission // hash => `*openSubmission`
 	log         *log.Entry
 }
 
diff --git a/services/horizon/internal/txsub/system.go b/services/horizon/internal/txsub/system.go
index 4a56e7cbc9..181a49f088 100644
--- a/services/horizon/internal/txsub/system.go
+++ b/services/horizon/internal/txsub/system.go
@@ -15,9 +15,10 @@ import (
 // Its methods tie together the various pieces used to reliably submit transactions
 // to a stellar-core instance.
 type System struct {
-	initializer    sync.Once
-	tickInProgress bool
+	initializer sync.Once
+
 	tickMutex      sync.Mutex
+	tickInProgress bool
 
 	Pending           OpenSubmissionList
 	Results           ResultProvider
@@ -73,12 +74,20 @@ func (sys *System) Submit(ctx context.Context, env string) (result <-chan Result
 	// check the configured result provider for an existing result
 	r := sys.Results.ResultByHash(ctx, info.Hash)
 
-	if r.Err != ErrNoResults {
+	if r.Err == nil {
 		sys.Log.Ctx(ctx).WithField("hash", info.Hash).Info("Found submission result in a DB")
 		sys.finish(ctx, response, r)
 		return
 	}
 
+	if r.Err != ErrNoResults {
+		sys.Log.Ctx(ctx).WithField("hash", info.Hash).Info("Error getting submission result from a DB")
+		sys.finish(ctx, response, r)
+		return
+	}
+
+	// From now: r.Err == ErrNoResults
+
 	curSeq, err := sys.Sequences.Get([]string{info.SourceAddress})
 	if err != nil {
 		sys.finish(ctx, response, Result{Err: err, EnvelopeXDR: env})
@@ -170,25 +179,40 @@ func (sys *System) submitOnce(ctx context.Context, env string) SubmissionResult
 	return sr
 }
 
+// setTickInProgress sets `tickInProgress` to `true` if it's not
+// `false`. Returns `true` if `tickInProgress` has been switched
+// to `true` inside this method and `Tick()` should continue.
+func (sys *System) setTickInProgress(ctx context.Context) bool {
+	sys.tickMutex.Lock()
+	defer sys.tickMutex.Unlock()
+
+	if sys.tickInProgress {
+		logger := log.Ctx(ctx)
+		logger.Info("ticking in progress")
+		return false
+	}
+
+	sys.tickInProgress = true
+	return true
+}
+
+func (sys *System) unsetTickInProgress() {
+	sys.tickMutex.Lock()
+	defer sys.tickMutex.Unlock()
+	sys.tickInProgress = false
+}
+
 // Tick triggers the system to update itself with any new data available.
 func (sys *System) Tick(ctx context.Context) {
 	sys.Init()
 	logger := log.Ctx(ctx)
 
 	// Make sure Tick is not run concurrently
-	sys.tickMutex.Lock()
-	if sys.tickInProgress {
-		logger.Debug("ticking in progress")
+	if !sys.setTickInProgress(ctx) {
 		return
 	}
-	sys.tickInProgress = true
-	sys.tickMutex.Unlock()
 
-	defer func() {
-		sys.tickMutex.Lock()
-		sys.tickInProgress = false
-		sys.tickMutex.Unlock()
-	}()
+	defer sys.unsetTickInProgress()
 
 	logger.
 		WithField("queued", sys.SubmissionQueue.String()).
@@ -199,6 +223,7 @@ func (sys *System) Tick(ctx context.Context) {
 		curSeq, err := sys.Sequences.Get(addys)
 		if err != nil {
 			logger.WithStack(err).Error(err)
+			return
 		} else {
 			sys.SubmissionQueue.Update(curSeq)
 		}
@@ -229,6 +254,7 @@ func (sys *System) Tick(ctx context.Context) {
 	stillOpen, err := sys.Pending.Clean(ctx, sys.SubmissionTimeout)
 	if err != nil {
 		logger.WithStack(err).Error(err)
+		return
 	}
 
 	sys.Metrics.OpenSubmissionsGauge.Update(int64(stillOpen))
diff --git a/services/horizon/internal/txsub/system_test.go b/services/horizon/internal/txsub/system_test.go
index 5257e15402..3276a6a78f 100644
--- a/services/horizon/internal/txsub/system_test.go
+++ b/services/horizon/internal/txsub/system_test.go
@@ -3,6 +3,7 @@ package txsub
 import (
 	"context"
 	"errors"
+	"fmt"
 	"testing"
 	"time"
 
@@ -52,9 +53,9 @@ func (suite *SystemTestSuite) SetupTest() {
 		Err: ErrBadSequence,
 	}
 
-	suite.sequences.Results = map[string]uint64{
-		"GBRPYHIL2CI3FNQ4BXLFMNDLFJUNPU2HY3ZMFSHONUCEOASW7QC7OX2H": 0,
-	}
+	suite.sequences.On("Get", []string{"GBRPYHIL2CI3FNQ4BXLFMNDLFJUNPU2HY3ZMFSHONUCEOASW7QC7OX2H"}).
+		Return(map[string]uint64{"GBRPYHIL2CI3FNQ4BXLFMNDLFJUNPU2HY3ZMFSHONUCEOASW7QC7OX2H": 0}, nil).
+		Once()
 }
 
 // Returns the result provided by the ResultProvider.
@@ -117,6 +118,48 @@ func (suite *SystemTestSuite) TestTick_Noop() {
 	suite.system.Tick(suite.ctx)
 }
 
+// TestTick_Deadlock is a regression test for Tick() deadlock: if for any reason
+// call to Tick() takes more time and another Tick() is called.
+// This test starts two go routines: both calling Tick() but the call to
+// `sys.Sequences.Get(addys)` is delayed by 1 second. It allows to simulate two
+// calls to `Tick()` executed at the same time.
+func (suite *SystemTestSuite) TestTick_Deadlock() {
+	secondDone := make(chan bool, 1)
+	testDone := make(chan bool)
+
+	go func() {
+		select {
+		case <-secondDone:
+			// OK!
+		case <-time.After(5 * time.Second):
+			assert.Fail(suite.T(), "Timeout, likely a deadlock in Tick()")
+		}
+
+		testDone <- true
+	}()
+
+	// Start first Tick
+	suite.system.SubmissionQueue.Push("address", 0)
+	// Configure suite.sequences to return after 1 second in a first call
+	suite.sequences.On("Get", []string{"address"}).After(time.Second).Return(map[string]uint64{}, nil)
+
+	go func() {
+		fmt.Println("Starting first Tick()")
+		suite.system.Tick(suite.ctx)
+		fmt.Println("Finished first Tick()")
+	}()
+
+	go func() {
+		// Start second Tick - should be deadlocked if mutex is not Unlock()'ed.
+		fmt.Println("Starting second Tick()")
+		suite.system.Tick(suite.ctx)
+		fmt.Println("Finished second Tick()")
+		secondDone <- true
+	}()
+
+	<-testDone
+}
+
 // Test that Tick finishes any available transactions,
 func (suite *SystemTestSuite) TestTick_FinishesTransactions() {
 	l := make(chan Result, 1)
diff --git a/services/horizon/internal/txsub/test_helpers.go b/services/horizon/internal/txsub/test_helpers.go
index 00b39791ac..2f866b3e1f 100644
--- a/services/horizon/internal/txsub/test_helpers.go
+++ b/services/horizon/internal/txsub/test_helpers.go
@@ -8,6 +8,8 @@ package txsub
 
 import (
 	"context"
+
+	"github.com/stretchr/testify/mock"
 )
 
 // MockSubmitter is a test helper that simplements the Submitter interface
@@ -43,11 +45,11 @@ func (results *MockResultProvider) ResultByHash(ctx context.Context, hash string
 // MockSequenceProvider is a test helper that simplements the SequenceProvider
 // interface
 type MockSequenceProvider struct {
-	Results map[string]uint64
-	Err     error
+	mock.Mock
 }
 
 // Get implements `txsub.SequenceProvider`
-func (results *MockSequenceProvider) Get(addresses []string) (map[string]uint64, error) {
-	return results.Results, results.Err
+func (o *MockSequenceProvider) Get(addresses []string) (map[string]uint64, error) {
+	args := o.Called(addresses)
+	return args.Get(0).(map[string]uint64), args.Error(1)
 }
diff --git a/services/horizon/main.go b/services/horizon/main.go
index 258e78fcc3..022e2ab809 100644
--- a/services/horizon/main.go
+++ b/services/horizon/main.go
@@ -9,6 +9,7 @@ import (
 	"github.com/sirupsen/logrus"
 	"github.com/spf13/cobra"
 	"github.com/spf13/viper"
+	"github.com/stellar/go/network"
 	"github.com/stellar/go/services/horizon/internal"
 	"github.com/stellar/go/support/log"
 	"github.com/throttled/throttled"
@@ -180,7 +181,7 @@ func init() {
 
 	rootCmd.PersistentFlags().String(
 		"network-passphrase",
-		"",
+		network.TestNetworkPassphrase,
 		"Override the network passphrase",
 	)
 
@@ -239,6 +240,10 @@ func initConfig() {
 		stdLog.Fatal("Invalid config: stellar-core-url is blank.  Please specify --stellar-core-url on the command line or set the STELLAR_CORE_URL environment variable.")
 	}
 
+	if viper.GetString("network-passphrase") == "" {
+		stdLog.Fatal("Invalid config: network-passphrase is blank.  Please specify --network-passphrase on the command line or set the NETWORK_PASSPHRASE environment variable.")
+	}
+
 	ll, err := logrus.ParseLevel(viper.GetString("log-level"))
 
 	if err != nil {
@@ -299,6 +304,7 @@ func initConfig() {
 		LogLevel:               ll,
 		LogFile:                lf,
 		MaxPathLength:          uint(viper.GetInt("max-path-length")),
+		NetworkPassphrase:      viper.GetString("network-passphrase"),
 		SentryDSN:              viper.GetString("sentry-dsn"),
 		LogglyToken:            viper.GetString("loggly-token"),
 		LogglyTag:              viper.GetString("loggly-tag"),