He's done that one several times and it makes me cringe – but its a hideously common error.

I agree – after his little obsessive rants about the use of 'irony' and 'who vs. whom' it does rather seem out of character. Perhaps it's time for Kate to correct him? She clearly has had a first rate education too.

quick, shoot me – i wrote "its a hideously common error". talk about boo boos. I should have written "it's a hideously common error" Not quite as horrible a mistake as your vs you're but bad never the less.

If even one of his parents were teachers, then he has to have had that particular drill driven into him. I can still hear my mother's "not I, dear, me!" speech.

There are a lot of grammatical errors I've noticed. A big one being that they often use "that" when they should use "who." Such as when Castle was talking about Kyra and he said "She was the one that got away." Kyra is a person, so Castle should have said "She was the one who got away." If she was a fish he was trying to catch or a great deal he was trying to close, then "that" would have been correct. That's not the only time but it's the one that stands out most in my mind right now.