It’s interesting that you brought up transaction isolation level.

There is a nasty gotcha to watch out for if your application is doing connection pooling: The transaction isolation level does not get reset by sp_reset_connection, so the transaction isolation level carries over to the next time the connection is reused. So if you run a serializable transaction the transaction isolation level will still be serializable the next time the connection is used unless it is specifically changed.

I spent a lot of time tracking that down before I realized what was going on. Microsoft has stated that this is not a bug and the behavior is intentional.