New captain's first day on an old ship. His very first order is that all hands should start scrubbing the rust off of the hull. First officer comes up and says "Captain, you don't want to do that. Rust is holding this ship."

Now why is this a programming joke?

Once you try to fix old code, it starts falling apart. You start believing in miracles, because there's no other explanation - the code did work at some time, right?