Grammar is a biggie!!! I usually can figure out what a person is trying to say, but sometimes I just get totally lost. I understand that not everyone speaks fluent english, and is not from the US, so I just try to filter through what they are saying, and see what they mean instead.
Spelling I can usually understand. Except when it's something that is from a command error. Then I don't know if it's a spelling error, or a system error. But I usually use the spelling check at the bottom just to double check, especially if I am up really late.