Transaction not connected, or was disconnected error

I recently came across this rather bizarre error. All I needed to do for my work was to import some files into my system. The import process used bulk insert along with some other processing, all inside a transaction.

When the bulk insert is successful, it works great. However, when the bulk insert fails, it results in "Transaction is not connected, or was disconnected" error, when the transaction is rolled back.

The stacktrace for the error is:-

 at NHibernate.Transaction.AdoTransaction.CheckNotZombied() at NHibernate.Transaction.AdoTransaction.Rollback() 

This in itself was not too helpful. However, when I had a look at the NHibernate source code, I had a better idea of what was going on beneath the hood. The definition for the CheckNotZombied() method reveals the clue.

private void CheckNotZombied()
{
if (trans != null && trans.Connection == null)
{
throw new TransactionException("Transaction not connected,                                                or was disconnected");
}
}


Proceeding further, the CheckNotZombied() method is actually called from a couple of places, Committ() and Rollback().
The code for either of these two looks like this :-
                                
                                //do stuff
                                CheckNotDisposed();
CheckBegun();
CheckNotZombied();
                                //do stuff


So, it looked like when the bulk insert query fails, it closes the transaction's connection, which resulted not only this error, but also the start of my very own blog).
A little googling revelaed that bulk insert is actually not transactional, and that sql server by itself will roll back the batches,as mentioned here. 
The other important point to notice is that when an exception occurs in the bulk insert part, sql server closes the connection. Interestingly, if you decide to catch the exception, and for some reason decide to go ahead with the next bulk insert, sql server will use implicit transactions for that.