stateful session beans are always tied to a specific client for which it is created for. so if the tran is left open it is beleived that the same bean will get to close/continue the tran(thought it is not the safest thing to beleive that the tran will be ended for sure)
stateless seeeion beans on the contrast are not tied to a specific client. once it completes serving a client (ie a business method completes) it can be assigned to any other client. so if it has open transactions... 1. that transaction may hang out there if that instance didnt serve any other bean or 2. the subsequent calls didnt call the method that closed the transaction.