What grammatical errors really piss you off?

[paperjam]

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Punctuation misuse really bothers me actually [mostly commas but it extends beyond that].

And that loose-lose thing mentioned earlier.

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I think I know why you're confused. It's because the other night when we were hanging out, and I told you "Your mom is a big loser," I think what you heard was "Your mom is big and loose."

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Thats true - but I suppose its because your mom is bigger and looser.

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You mean "it's," not its and loser, not looser. Oh you knew that and are trying to level me. You owned me!