NHibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session: 0, of class: Foo.Entity.Bar
   at NHibernate.Impl.SessionImpl.DoSave(Object obj, Object id, IEntityPersister persister, Boolean useIdentityColumn, CascadingAction cascadeAction, Object anything)
   at NHibernate.Impl.SessionImpl.SaveWithGeneratedIdentifier(Object obj, CascadingAction action, Object anything)
   at NHibernate.Impl.SessionImpl.Save(Object obj)

I didn't see much out there on this, but Geoff Lane picked up on it pretty quickly. It turns out that my Bar.hbm.xml mapping file had set the id generator incorrectly to "generated", which was a problem because it was actually a MSSQL IDENTITY field. The mapping should have looked like this:

<id name="Id">
  <generator class="native" />
</id>

Luckily for me, I had run my tests by trying to insert 2 entities inside the session so the error bubbled up right away.