Working a a GIS company, which just makes this so much worse:
Had to do conversion of wind direction degrees to shorthand symbols (N, SE, etc). Everything ends up rotated by 90 degrees, and the project manager craps on the dude that wrote the API I get the degrees from. He claims innocence, so I get it next.

Turns out, I used the Cartesian system (where 0 starts on the right) instead of the cardinal directions (where 0, North, is at the top).

Yeah, still haven’t lived that one down...