Spelling is my ultimate nemesis (as I have discussed many, many times), but seriously - there are a few words that are now part of my vocabulary that Microsoft Word hasn't bothered learning yet. Maybe this means they aren't actual Engligh language words - but I use them constantly, so they are words (I am just making up verbs here. Bomb. Watchmenow)
This means that I will write an email/proposal/letter and I will get that little red line of failure and I throw my hands up in protest (although, I finally added many words to the Microsoft word dictionary, my computer still doesn't recognize them.
Its an epic battle and it keeps me nice and enraged when I am still at work at 8 PM.