Notice: Undefined variable: CRM16923AnUnreliableMethodHasBeenUserToDeterminePaymentProcessorFromEvent
Notice: Undefined variable: CRM16923AnUnreliableMethodHasBeenUserToDeterminePaymentProcessorFromEvent in CRM_Contribute_BAO_Contribution->loadRelatedObjects() (line 2796 of .../CRM/Contribute/BAO/Contribution.php).
It's not clear if this is an expected notice that's on the roadmap already someday to improve, or if this is an unexpected path that should no longer be possible.
I'm not sure exactly what the trigger is yet, but it can be reproduced on dmaster.demo with contact id 9 and contribution id 46 by just opening the contribution and saving it (turn off popups so you can see the error). The contribution looks like this: