Spellcheck - spoiler alert - only checks for spelling errors. (If only they could have named it in some way to clue users in on that, right?)
This means that a word that is technically spelled correctly - and might even look right at a glance - isn't necessarily the word that you intended. But Spellcheck isn't going to tell you, because Spellcheck is looking at the spelling of the word, and not the word's meaning.
This is how men end up with receding heir lines, and why women have to be careful about how they style their hare.
Spellcheck, unfortunately, is also the giver of mirth to many editors, because of what it leaves behind.
Just last week I came across this gem:
"The cow towing attorneys answered his every whim."
When I first read that, I inserted a mental hyphen into "cow-towing" and assumed that all of the attorneys were being forced to pull cattle along behind them.
A friend of mine pointed out, however, that without the hyphen this would mean that the attorneys were being towed by a cow while said cow was responding to all of the whims.
I honestly had to read the sentence multiple times before I realized that the intent was to state that the attorneys were "scraping and grovelling" before the man in question (aka "kowtowing"), and there were no bovines in the room.
Personally, I think Spellcheck was just having too much fun with that one and decided to milk the situation for laughs