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.
Understand there are several types of editors. Just because you hire one, doesn’t mean they’ll do the job you expect, especially if you don’t understand there are different types. They may do a great job within their realm, yet miss other problems. I can’t tell you how many problems I’ve found in books where the author supposedly hired an “editor.”
Here’s the basic run-down: Proofreaders look for typos. Copy editors look at punctuation and grammar. Line editors look at everything. Content editors look at plot & characterizations. If this is news to you, then I suggest you read this great article that gives more detail.
If something throws you out of a story you’re reading, figure out why. Then make sure you’re not guilty of the same thing. You can learn from all writers, whether more or less skilled than you are. Typos are one thing that really jolt me out, though blatantly inaccurate science is a close second.