I once added a backdoor to gain full Admin rights (without corporate sign-off) to a site I was building for a relatively large corporation. Though this was only intended for QA and Dev, I forgot to get rid of this backdoor when I deployed to production, and it was shortly found. A lot of the data that had been input was mangled and broken (or just outright deleted) and not backed up, which forced a lot of the creative people to redo several days of work.

I was fortunate enough to have a boss was pretty understanding, and I was not fired, though I genuinely felt he would have been justified in doing so.