My brain is apparently blocking my own dumb errors, though I know I've made some, but none of mine top one from a predecessor at a former employer, a major PC manufacturer.

There was a subroutine which was supposed to adjust prices on certain items in certain circumstances; something to do with government contracts and pricing to the .1 cent level (yes, 1/10th of a cent).

Well, someone bozoed, something like = instead of += or adjusting the wrong variable, and was setting the price to a tenth of what it should be. And this somehow made it live. The "fix", which was in place when I came in, turned the whole subroutine into a no-op, by modifying one local variable and returning a different one.