While the occasionally error creeps in here, I think my brain came out wired in such a way that spelling, grammar, and punctuation come pretty naturally to me. Don't know if it's genetic or learned, but I don't consider this as having anything in particular to do with intelligence. Still, I have my peeves, because often it's just that people were never taught the rules.

Commas -- there are lots of situations where they're used, but my peeve is when people to use commas to separate two clauses (consisting of a subject and a verb). For example, "Don't separate clauses with a comma, it's not good practice." If you were to replace the comma with a period, you'd have two perfectly good sentences; therefore, the comma is the wrong choice. Use a semicolon or a dash instead. I fixed about a dozen of these in a document today.

It's versus its. I'll give you a rule, so that you'll never get it wrong again. I've used this since childhood. Picture the apostrophe in "it's" as the dot on a letter "i". This makes "it's" into "itis", or "it is". Now ask, does the sentence still make sense? If it does, leave the apostrophe. If it doesn't, get rid of it. Now, don't let me catch you doing it again.