Ever write an article and then after publication you notice a spelling error? It’s horrifying, I know! You think to yourself, “but I used spellcheck!” Well, I hate to break it to you, spellcheck is an indispensable tool, but it isn’t a fool-proof application.
It doesn’t correct words used incorrectly if you mean ‘their’ but use ‘there’ and spell it correctly spellcheck will leave your error there for the whole world to see.