Fairly often.
I tend to fork open-source stuff when I want to make my own adjustments, often the adjustments I make are not wanted upstream. So I'm doomed to having merge conflicts for all eternity.
Often what happens though is that I fix some bug, and then a few months later the upstream fixes it in some other way - so the conflict resolution is to basically just throw away my own (clearly superior but now obsolete*) changes, to avoid more conflicts later on.
(* I'm joking. But it does feel bad to throw away good work.)