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!