Browser does not always receive events. Could be caused by temporary disconnect.
Possible Solution:
Update client code to detect state of pusher connection and respond accordingly.
Server occasionally fails to send events due to inability to connect to pusher.
Possible Solution:
Retry these connections until confirmation of receipt is received.
This post describes an example of how to confirm receipt of messages: http://pusher.tenderapp.com/kb/faq-common-requests/what-happens-if-a-user-is-on-a-poor-connection-that-occasionally-drops-will-they-miss-messages