Biggest sin? Probably the database application with an Access backend. What can I say? I was young and foolish, and the client flat out insisted that Access was the only possible option they would accept.

No, wait, maybe that pales into insignificance in comparison to a certain mission-critical application implemented in, um, JavaScript... and I don't mean the socially acceptable AJAXy kind. (There were reasons, I swear! And it's very good JavaScript, with all kinds of clean abstraction and an exhaustive test suite and everything.)

Alas, I fear that I have perpetrated such unforgiveable acts that no amount of penance will now see me permitted entry through the Perly Gates...