There are two main reasons for this to show up.

NHibernate Warning Surfacing

The NHibernate Profiler will surface logged warnings from NHibernate, so you could figure out what was the reason for the warning and fix that.

NHibernate is quite good in detecting problematic situations and warning you about that, so please pay attention to whatever warning are surfaced.

NHibernate Error Detection

Whenever NHibernate is running into an error (transaction deadlock, optimistic concurrency exception, etc), the NHibernate Profiler will detect that and show you the full error (including the exception message and the stack trace).