Spell check as you go (f6) is a little flakey.

If you're working with spell-check-as-you-go on (f6), sometimes flagged words will go dark (which in my theme, means they are invisible), other times, they are white text (as they should be) with red underlines. If they do go dark, simply continuing to type will fix the problem, but it's a shame to have such a beautiful product marred by such a silly bug.