as someone who cannot spell (like, at a diagnosable level), one thing that annoys me with spellcheckers is how they don't distinguish "you spelled that wrong" and "that's not a word, according to some fancy dictionary"
@foone Other annoying problems: I've yet to encounter a spellcheck that lets me type "realise" without it marking it as wrong, or "correcting" it for me. I guess Merriam-Webster is more to blame for that one.