Licence / license, and practice / practise. I have to look them up every single time because I forget which of each is the noun and which is the verb, and even then, there are situations where using the noun as a verb might actually be the right thing to do and I hate the whole thing. So I probably still get those wrong whenever I use them.

Barring brain farts (increasingly common) and muscle memory leading me astray on the keyboard, my spelling is otherwise fairly good, but those pairings I could do without.