I liked the core of this question, and wanted to re-ask it in a way that made it less about 'fun' and more about 'What do these past mistakes tell us about how we can write and test software better?'

As an SDET, I'm always looking for anecdotes about new and interesting ways that programs can fail. I've learned a lot from these tales in the past, and would like to get that from the intelligent people in this community as well. I'd be interested in hearing what the issue was, how it was caught, if you think there was anything that could have reasonably done to catch it earlier or to avoid the same issue on later projects, and any other interesting lessons you took away from this bug. Please only write about bugs you personally were involved with, ideally on a project you worked on (e.g., no "10 years before I was born, this happened and it was FUNNY!" answers).

Please vote up answers that are thought-provoking or could change how you develop or test in some way, so this isn't just 'social fun'. Try to avoid voting up something just because it was funny.