The moment you think you’ve seen all possible NHibernate errors, a new one pops up 
A colleague asked me to have a look at a query he created using the QueryOver syntax. But instead of getting the correct results, the query failed with the following error message:
“Possible non-threadsafe access to the session.”
I was 100% sure that we were working on a single thread, so I had no idea what caused this error. Even stranger is that when I switched from an IStatelessSession to a normal ISession, the error disappeared…
The NHibernate JIRA brought the solution:
In a stateless session, queries load objects in a two-phase process: in the first phase, a temporary persistence context is populated with empty objects; in the second phase, the objects' member data is read from the database. If an object contains an association or a collection, the query performs a recursive call to the session's get() method. This clears the temporary persistence context.
If the parent object contained any other associations to be read as part of the second phase, Hibernate threw an assertion because the associations cannot be found in the persistence context.
This has been fixed by introducing a new method: <literal>org.hibernate.engine.PersistenceContext.isLoadFinished()</literal>. This method tells StatelessSession when to clean the temporary persistence context.
And indeed when I switched to the most recent NHibernate version, the problem was gone!