For some reason, I always seem to screw up the spelling of "misspell". I'm also rather paranoid of doing so, and never really trust the built-in spell-checkers in my applications. If I'm Internet-connected, I'll typically hit up dictionary.com, just to be sure.
It's probably safest to just use "spelling error", instead. Unfortunately, that requires restructuring the statement and often times compromises the flow of a written work.