I make frequent minor grammar and spelling mistakes. Spoken english would rarely pick them up, nor would the listener be genuinely confused, but written english is different and while our Chomsky grammer parser is just one brain, we hear and read things differently. It is and It(possessive)s are not the same in semantic intent, and even sometimes in spoken flow people have to ask "did you mean it is, or it possesses" in some manner.

Spelling checkers which cannot read do not help. here and hear are both legal in the sentence parse in some ways, so cannot be detected as the wrong form without a higher semantic model. Few systems have this. Therefore, many small mistakes can creep through, apart from the ones I mis-type the system may itself be making them, in ways which our own spellchecking brain do not pick up. Here what I say, Hear what I say...