I'd draw an entirely different lesson from this -- namely, killing time by doing "100 other minor cleanup changes to a bunch of different files" on a 40,000 LOC

I'd draw an entirely different lesson from this -- namely, killing time by doing "100 other minor cleanup changes to a bunch of different files" on a 40,000 LOC legacy production system that (apparently?) has no tests sounds like a Really Bad Idea(tm). Don't mess with code you don't understand if you don't have even the basic safety net of decent unit tests in place.