We all have them, usually they are homonyms, those words that we use incorrectly. To, too, two; there, their, they're. We can't seem to escape them, and the wrong one can make a huge difference in what you are trying to say.
Here are some that I look for when I CP and edit in my own writing:
rode -- road
then -- than (while not homonyms, I find many people flip these around)
past -- passed (I constantly ask my husband on this one)
one -- won
effect -- affect (I don't see this one much, but recently had it pointed out to me in some work things)
What troublesome words do you have to watch out for?