At a previous job we had a pretty serious off-by-one error in a crypto module. This module was distributed as a binary DLL to dozens of customers. For various reasons it would have been a costly and inconvenient project to rebuild it from source and redistribute it. So I located the bug and realized it could be fixed by just increasing a constant in the binary.
The best part is that the relevant bit pattern turned out to be legible in ASCII. So this serious bug could be fixed by searching for something like ".," and replacing it with ".~". I taught the devops guys to do this in Notepad++. The DLL could even be reloaded without restarting the server. Problem solved!
When I left the company and wrote a little report about this, the architect patted me on the back and sighed deeply. With his customary tone of affection and good cheer, he said "mbrock, you are a horrible, horrible person."