I make these all the time, and even the best spell checker won’t catch them, because the words are actually spelled correctly, but used incorrectly. This usually happens when I’m typing too fast. It’s not that I don’t know the grammar rules, it’s my fingers…it’s their fault.
For example, I’ll type you when I mean your, and your when I mean you: "Be sure to catch these mistakes by reading over you blog post before your save it."
Here’s a great review of Ten Common Writing Mistakes Your Spell Checker Won’t Find over on Judy Rose’s Writing English blog. It’s a good review of what to look for before you save and publish.