Oh, I have a couple of fun ones.

"The most impossible one" was during my first "professional" job, while still in college. I was writing part of the GUI of an app - a screen that showed a grid with controls and data. This was ~20 years ago, so C++ and MFC; the control was, IIRC, a regular listbox with some custom drawing.

We got a bug report from an user that was something like "if I do this and that, the empty cells become '3'". Some of my very senior colleagues had drilled into me that CUSTOMERS ALWAYS LIE, so my first reaction was to be sceptical. This was, after all, an impossible bug - most of these cells were absolutely empty (as in, it was impossible for them to have content), and the action the user was describing was absolutely unrelated to this control anyway.

So I tried to reproduce the bug, and to my utter disbelief, the blank cells now showed a "3".

After much debugging, I found the root cause. I was using some API call to lock the char* buffer of an entirely unrelated CString, for some direct manipulation. IIRC the second argument was the length of the string, but there was the bug: I incorrectly thought this arg was the offset, not the length, so I was passing 0 there.

Somehow MFC must have been doing something like interning strings, because by locking this CString of size 0, MFC was giving me a pointer to the char buffer _of the application-wide empty string_ - and I was writing a "3" there for unrelated reasons. So every empty string in the program (or at least for some controls) now was "3".

The fix was trivial once I identified the bug, but I always remember this as the most impossible "can't happen" bug that did, in fact, happen.

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"The funniest bug title" was "HANDBAG TURNS INTO A BANANA". By then I was making games (http://www.mysterystudio.com). This was one of the Sherlock games, IIRC, of the "hidden object" genre - there's a list of objects you have to find in a very cluttered scene. When you click on the object, it flies to your inventory. In particular, if the object was partially obscured by the scenery or another object, it would appear unobscured (think "in the foreground") while it flew to the inventory.

To do this, we had a static background with a bunch of "filler" objects, but no "target" objects. Then, for each "target" object, we had a partial (P) and a full (F) image. We'd show the P image of every object, then when the player clicked it, we'd show the F image and animate it to the inventory.

What the tester was reporting was that there was a handbag that, when clicked, turned into a banana.

The root cause was again trivial. For some reason, the artist had made a naming mistake while exporting assets, so we had a P asset of the handbag, but its matching F asset was a banana.

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"The most difficult to debug" was at Improbable. In the very early days, part of the world simulation code used Scala and Akka, with a pubsub system (this has been rewritten using more sensible technologies and architectures). During one of our nightly stress tests, we had a bunch of entities essentially walking the extents of the world, looping endlessly for hours. And after hours of running, some entities (but not all) would get stuck, as in "stopped moving", at the boundary of two contiguous simulation regions (still very likely in the same machine).

What followed was a three-week investigation that truly tested my sanity. This was a massively distributed system, and the bug was very hard to reproduce. I spent countless hours staring at logs, diffing logs from "good" runs and "bad" runs, trying to reduce the scenario to isolate the bug. Everything was suspect at some point or other, from our pubsub implementation, to the implementation of hash maps in the Scala standard library.

In the end it turned out to be an extremely subtle race condition in some cache somewhere in the pubsub subscription and desubscription logic. It took 3 weeks of suffering, doubting my own sanity, and being tempted to abandon it (at that point, we were considering just rewriting a big chunk of the code, just to nuke this bug from orbit). I'm glad I persevered and fixed it - it would have haunted me to this day if I hadn't. I still remember its name, and I probably always will. F* you, ENG-168.