We've had some pretty extensive rewriting going on for the last few months in one of the big customer projects (the VoIP billing system). Spending three or four days in here stabilising the code base, tracking down all the little refactoring errors that didn't get caught with the test suite, fixing bugs that fell into the "we don't need that feature now, leave it broken" bin, that kind of thing.

There haven't actually been that many bugs, but getting rid of them now makes me happier than having to deal with them in the middle of continued refactoring and development.