So I know Notch publicly stated that he hadn't written tests for the Minecraft code and didn't intend to because it's too hard to do, but the problem (other than your mind blowing when you read things like that) is that every release introduces the most stupid regressions possible. Let's take the recent 1.2 set of releases as the example here:
1.2.1: After a lot of preview builds, we have a whole mess of bugs. Fair enough, there are a lot of changes. One of these is that big trees are far rarer than they're supposed to be.
1.2.2: Big tree generation is fixed. The game now crashes if you look upwards at the sky, and mobs don't burn in sunlight anymore.
1.2.3: Mobs burn in sunlight again. Silk touch no longer works on glass.
These aren't even related things. I can't imagine there being any shared code between the thing being fixed and the thing that gets broken. At least Jeb is actually trying to improve the game, but seriously man... get some regression testing implemented. Let this be a lesson to you all.