Here is the situation I'm faced today:

I have a jsp page with a form on it and an a4j:commandButton to perform some ajax stuff.

I've performed a huge refactoring and have made a mistake that resulted in errors while submitting the form (one field was populated from combobox that had values of an inappropriate type). Since the form hasn't buttons that submits it directly (the form only updates on ajax actions) I didn't notice the error. The only thing I noticed was my ajax actions didn't do anything.

The only log entry I've found was the following:
"FacesMessage(s) have been enqueued, but may not have been displayed." and it had log level INFO. IMHO it isn't enough for this situation