There's a corollary rule about optimizing performance last. Somehow, I never met a PM that knew about budgeting time in the schedule for that part...
For a while I was tasked with maintaining a VB6 program that was about 6k lines of Chef Boyardee code in a single file. I think there had been about a dozen guys there before me and there were a lot of comments in there like:
'Don't you @#$!-ing dare touch this line of code!!
'I can't figure out what it does but everything blows up without it