It seems that whenever NHibernate gives an Exception, I get the error message
"null id in <ClassName> entry (don't flush the Session after an exception occurs)". As far as I can tell, this will occur for any failed sql insert statement whether an identity column isn't set, a column name is incorrect, string or binary data would be truncated, inserting null into a non-null column, or any other SQL Server level exception.

The exception has no inner exception and a useless error message. Is there any way to get the actual error message for the exception (the original "String or Binary data would be truncated")?

I'm using NHibernate 2.1.1 GA.

Is that the first exception you are getting ? As soon as you get an exception in a session, the session becomes useless. You have to dispose the session and create a new one. If you try to flush a session after an exception you will get the exception you got.