Be aware of your most common typos. Mine are typing “you” instead of “your” or “the” instead of “that”. A simple spellchecker is very likely to miss such goofs when it’s an actual word and not misspelled, just not correct in context. These are also difficult to find when you’re proofreading or editing because that same disconnect that originated between your brain and fingers will come back to haunt you when reading it. However, an alert reader will trip over it in a heartbeat. During your final edit, be sure to take your time and read each word deliberately, looking for such things. If you’re beta reading for another author, be sure to point out such goofs because the author is less likely to catch it.