My worst bug(s) is(are) related to the TopLink ORM.
In one particular case, we were working on a *huge* system and different "modules" were assigned to different teams. Every module was working perfectly by itself (as confirmed by the SUnit testing) but sometimes, the database would get out of whack.
I finally realized that depending on the classes you used to perform object deletion (we had a parent-children-familymembers kinda model), none of the "modules" did it the same way and they all differed when it came to access the to-be-deleted object. Most classes accessed the same "kind" of object from a single access point but not all of them! That did create interesting problems where the to-be-deleted object reappeared in the database with old values, or just vanished during some operation only to reappear afterwards... Besides, debugging proxies and cached objects is a real pain in the *ss ! And in many cases, you can't ever log properly anything as it would instantiate the proxies! So I ended up not trusting the Smalltalk side of things and worked my way out from DB/2 logging only!
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Beno��t St-Jean
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"A standpoint is an intellectual horizon of radius zero". (A. Einstein)