diff --git a/configmap/loglevel_change_test.go b/configmap/loglevel_change_test.go
index 4e8a658..366f7ea 100644
--- a/configmap/loglevel_change_test.go
+++ b/configmap/loglevel_change_test.go
@@ -36,13 +36,20 @@ func TestDynamicLogLevel(t *testing.T) {
 	if err = os.Mkdir(pDir, 0o755); err != nil {
 		t.Fatalf("unable to make %v: %v", pDir, err)
 	}
+	fName := path.Join(pDir, "extra_flag")
+	if err = os.WriteFile(fName, []byte("ignored"), 0o644); err != nil {
+		t.Fatalf("unable to write %v: %v", fName, err)
+	}
 	var u *configmap.Updater
 	log.SetLogLevel(log.Debug)
 	if u, err = configmap.Setup(flag.CommandLine, pDir); err != nil {
 		t.Fatalf("unexpected error setting up config watch: %v", err)
 	}
 	defer u.Stop()
-	fName := path.Join(pDir, "loglevel")
+	if u.Warnings() != 1 {
+		t.Errorf("Expected exactly 1 warning (extra flag), got %d", u.Warnings())
+	}
+	fName = path.Join(pDir, "loglevel")
 	// Test also the new normalization (space trimmimg and captitalization)
 	if err = os.WriteFile(fName, []byte(" InFO\n\n"), 0o644); err != nil {
 		t.Fatalf("unable to write %v: %v", fName, err)
diff --git a/configmap/updater.go b/configmap/updater.go
index 28011d9..76cf152 100644
--- a/configmap/updater.go
+++ b/configmap/updater.go
@@ -12,6 +12,7 @@ import (
 	"os"
 	"path"
 	"strings"
+	"sync/atomic"
 
 	"fortio.org/dflag"
 	"fortio.org/dflag/dynloglevel"
@@ -38,6 +39,7 @@ type Updater struct {
 	watcher    *fsnotify.Watcher
 	flagSet    *flag.FlagSet
 	done       chan bool
+	warnings   atomic.Int32 // Count of unknown flags that have been logged (increases at each iteration).
 }
 
 // Setup is a combination/shortcut for New+Initialize+Start.
@@ -124,9 +126,13 @@ func (u *Updater) readAll(dynamicOnly bool) error {
 			continue
 		}
 		fullPath := path.Join(u.dirPath, f.Name())
+		log.S(log.Debug, "checking flag", log.Str("flag", f.Name()), log.Str("path", fullPath))
 		if err := u.readFlagFile(fullPath, dynamicOnly); err != nil {
 			if errors.Is(err, errFlagNotDynamic) && dynamicOnly {
 				// ignore
+			} else if errors.Is(err, errFlagNotFound) {
+				log.S(log.Warning, "config map for unknown flag", log.Str("flag", f.Name()), log.Str("path", fullPath))
+				u.warnings.Add(1)
 			} else {
 				errorStrings = append(errorStrings, fmt.Sprintf("flag %v: %v", f.Name(), err.Error()))
 			}
@@ -139,6 +145,11 @@ func (u *Updater) readAll(dynamicOnly bool) error {
 	return nil
 }
 
+// Return the warnings count.
+func (u *Updater) Warnings() int {
+	return int(u.warnings.Load())
+}
+
 func (u *Updater) readFlagFile(fullPath string, dynamicOnly bool) error {
 	flagName := path.Base(fullPath)
 	flag := u.flagSet.Lookup(flagName)
diff --git a/go.mod b/go.mod
index dc96a2d..8f8f9c8 100644
--- a/go.mod
+++ b/go.mod
@@ -7,7 +7,7 @@ require (
 	fortio.org/log v1.11.0
 	fortio.org/sets v1.0.3
 	github.com/fsnotify/fsnotify v1.6.0
-	golang.org/x/exp v0.0.0-20230321023759-10a507213a29
+	golang.org/x/exp v0.0.0-20231006140011-7918f672742d
 )
 
-require golang.org/x/sys v0.4.0 // indirect
+require golang.org/x/sys v0.13.0 // indirect
diff --git a/go.sum b/go.sum
index 925d6a6..91a8259 100644
--- a/go.sum
+++ b/go.sum
@@ -6,8 +6,8 @@ fortio.org/sets v1.0.3 h1:HzewdGjH69YmyW06yzplL35lGr+X4OcqQt0qS6jbaO4=
 fortio.org/sets v1.0.3/go.mod h1:QZVj0r6KP/ZD9ebySW9SgxVNy/NjghUfyHW9NN+WU+4=
 github.com/fsnotify/fsnotify v1.6.0 h1:n+5WquG0fcWoWp6xPWfHdbskMCQaFnG6PfBrh1Ky4HY=
 github.com/fsnotify/fsnotify v1.6.0/go.mod h1:sl3t1tCWJFWoRz9R8WJCbQihKKwmorjAbSClcnxKAGw=
-golang.org/x/exp v0.0.0-20230321023759-10a507213a29 h1:ooxPy7fPvB4kwsA2h+iBNHkAbp/4JxTSwCmvdjEYmug=
-golang.org/x/exp v0.0.0-20230321023759-10a507213a29/go.mod h1:CxIveKay+FTh1D0yPZemJVgC/95VzuuOLq5Qi4xnoYc=
+golang.org/x/exp v0.0.0-20231006140011-7918f672742d h1:jtJma62tbqLibJ5sFQz8bKtEM8rJBtfilJ2qTU199MI=
+golang.org/x/exp v0.0.0-20231006140011-7918f672742d/go.mod h1:ldy0pHrwJyGW56pPQzzkH36rKxoZW1tw7ZJpeKx+hdo=
 golang.org/x/sys v0.0.0-20220908164124-27713097b956/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.4.0 h1:Zr2JFtRQNX3BCZ8YtxRE9hNJYC8J6I1MVbMg6owUp18=
-golang.org/x/sys v0.4.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.13.0 h1:Af8nKPmuFypiUBjVoU9V20FiaFXOcuZI21p0ycVYYGE=
+golang.org/x/sys v0.13.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=