One that always irks me to no end, is every time I see someone ham fisting csv handling by hand instead of using an established, well-sourced library. They almost always fail at commas or newlines in quoted text... It's one of the more annoying things.

Currently working on replacing a couple decades old system, and my csv output is using a library that isn't quoting all the strings that don't require quotes... so I'm forced to do it (for compatibility) with the other system this csv is going to. (sigh).