There's a debate going on right now on python-dev about what kind of changes are acceptable in micro releases. (Micro releases are where you go from x.y.z to x.y.z+1 - for example, from 2.2.1 to 2.2.2.)

At one point, Fred Drake said "We might not be able to remove a core dump since it would allow code to run that was not run before, thereby changing the behavior of the code".

No one would rely on something like a core dump, right? Well, perhaps, but I was reminded of one of my nastiest ever iSeries hacks, which was very nearly as bad. Scarily enough, this was (and is) used in production code.