Most notably, it's possible for a test to fail an assertion, meaning that the test method is exited, while there are still pending SQL statements to be flushed.
The current clean up processing (in the IsisTransactionRule) just does a close session, and the first thing that does is close the current transaction if any, by (attempting to commit). However, if that commit in turn throws an exception, then remaining close tran/close session functionality is skipped. The net result is that the transaction is left incomplete (transactionLevel=1) with the currentTransaction in a state of MUST_ABORT.
Every subsequent test, when it attempts to start its own transaction, immediately finds there's already a MUST_ABORT transaction in place, and fails immediately.
The net effect is that, once one test has failed in this way, then all other tests in the test suite will also fail.
The fix is just to do the clean up more robustly in IsisTransactionRule.