Which security problems do you loathe dealing with in your PHP code?

Application security is very much one of those you love it or you hate it topics for most of us.

But wherever you sit, there’s probably a problem (or superset of distinct problems) that you find vexing to deal with.

I’d like to hear about what those topics within security are, and why they annoy you.

(This thread may or may not lead to the development of one or more open source projects.)