Of all the bugs I've came across I'll never forget this one because it was so severe and easy to miss. Later, when I was a Test Lead it was the first thing I added to my sign-off checklist… A few years ago, our product was practically out the door when we learned that setup would fail if the computer clock was fast forwarded. Basically, a year or so after shipping the product would no longer install, because some time-bomb code remained in the setup custom actions. The team had performed timebomb tests. But did them by moving the clock forward *after*, and not before, installing the product. Easy mistake to make, if you aren't aware of the setup timebomb or that setup can have custom actions. For me, it's a good illustration of a common reason why teams miss bugs: knowledge/communication gaps.
Do you have a bug whose story you love to tell? Let me know!