Well, this had better be my last post of the night. I've got to be at work in about 10 hours...
Back at Microsoft, we had six major milestones at the end of a project: code complete, go/no-go, content complete, ZBR, release candidates and gold master. Code complete is roughly equivalent to Alpha in most other companies, and Content complete is roughly equivalent to Beta.
ZBR stood for "Zero Bug Release," and it meant that there was no active bug in the database older than a short period of time (depending on the project, it ranged from 24 to 96 hours). It was intended to mean that not only had all older bugs have been handled (either fixed or triaged out of consideration), but that the bugs were also being fixed essentially in real-time as they were found.
After ZBR, there was always a brief period before release candidates started for "stabilization purposes." In other words, tons of additional bugs were usually found after ZBR had been hit, and the extra time was necessary to whittle those down to a release level. This bubble of additional bugs got to the point where we were nicknaming it the "Zero Bug Bounce" or ZBB.
So why is there a ZBB? Surely those bugs existed before ZBR. Most of them did. Some of the bugs come about as a result of the speedy bug fixes leading up to ZBR. A lot are just a result of the team being "used" to the way the program runs, but when ZBR hits, everyone goes into "last chance to find bugs" mode. It doesn't matter what it is, you want it found, reported, properly triaged and hopefully fixed.
So you down some extra Mountain Dew, stare at the monitor for a few hours, invariably end up entering some duplicates of some issues that were entered back around Milestone 2, and finally say you've found all you can. They get triaged or fixed, and then the Release Candidates come forth...
In other words, the ZBB doesn't come about as part of an inadequacy in the testing of your product, but instead as a result of the rush that comes from seeing the end in sight. ZBB is test's second wind, the last ounce of strength expended at the end of a marathon needed to reach the finish line.
[Edit: Fixed multiple typos and grammar errors. Why the Hell to I post at 12:30am?]