Imagine my delight when I found this mistake in an advertisement in Parents magazine. The headline uses the singular antecedent student and the plural pronoun their. I understand why it would be politically correct to use the pronoun their because his/her is obnoxious and using just his or just her would be selectively choosing one gender or another. This could have been written with the plural antecedent students.
When I showed this to some of my classes, several students caught this mistake. Woo-hoo!

Another sign from the same student...I think that "giftshop" should be 2 words, and "Bills" should be "Bill's."

I student of mine e-mailed this one to me. I think these mistakes are intentional, and the grammar nerd in me gets a kick out of this. :)

I found this one at the Saline County Fair. "Teardown" is not one word. It should be written as two words.

This is from a gas pump display. If I understand this one properly, the apostrophe is unnecessary. If they mean that the magazine IS inside, then maybe this would work. Ahhh! The apostrophe is just unnecessary.