It occurs to me that a few places in my code effectively have 'backup comments', but invariably because I found a faster or otherwise apparently more desirable way to implement something and having the original version Right There both acts as an explanation of the current code and also reduces the effort required to back out to the previous implementation temporarily to double check parity.

I'm ... invariable a little ambivalent about doing that, but I do it rarely enough and the times I have have come in handy often enough later that I think it's one of those things where "ask yourself twice if you really want to do this, but don't never do it" applies.