I've both responded to SO questions with updates, as well as had one of my top answers be replaced by a newer, better solution. In browsing questions, I've also seen a number of updates to existing answers.

I guess the point is, if you've spent a lot of time figuring something out, and you've proved that a number of solutions on the internet are now wrong, you're tempted to make it easier for the next guy. I think this is true regardless of the medium, but SO seems to have done well at creating an environment where it happens.