Things change all the time. People, including managers, are human and specs aren't something that always can be "carved in stone" in advance and carried out to perfection.

There certainly are pathological situations where people really do need to leave a mark and that's why "the duck" technique that Rachel described works.

This pops up in presentations too. Sometimes you know in advance that your presentation is going to have an audience with one or more "well, actually..." types who will try to poke holes a little too aggressively in your argument.

One technique for handling this is to deliberately leave out some persnickety detail in your deck, but cover it exhaustively in an auxiliary slide that you don't show unless someone points out the omission. It works great. Every. Time.