Any grammar rules you wish you had never learnt

After the apostrophe, the thing that most annoys me is misuse of the words "fewer" and "less". I can't stop myself from correcting anyone on TV who says something like, "Less people voted in this election", and I stand in the supermarket glaring at the "ten items or less" signs over the tills. The thing is, I don't know why it annoys me so much. I didn't know the rule myself until I was in my thirties and did some TEFL modules as part of my degree in modern languages, and I have come to the conclusion that people teaching English as a foreign language are about the only people - apart from their students - who are aware that such a rule exists.

Logically, I think that if 99% of the population is unaware of a grammatical rule, then that rule is not worth the paper it is written on, and we should strike it out of the grammar boods. So why do I get so hot and bothered every time I encounter it?