When testing illegal sequences of operations, I call timestamp_transaction with commit_timestamp=0.  That returns an error, but then in the same transaction, if I call prepare_transaction with a legal timestamp I see either:

• An ASSERT in diagnostic mode (see test_timestamp_bugx.py attached).
• Success returned in one testcase (see test_timestamp22.py, soon to be merged).
• Error returned in one testcase (see test_timestamp_bugx.py attached).

If we've marked the transaction in error so that the commit is not allowed, I think I would expect an error from prepare with any input.  Generally, we should not ASSERT for cases of illegal input.

To see the error, remove the statement marked with comment "AVOID ASSERT" in test_timestamp22.py.