Why do I keep hearing this?

Even though I never had Mr. Dugan as a teacher at Dodd Jr. High... i was familiar with one of his sayings (one that no doubt, many other teachers and grammarians have uttered over the years):

Never use a preposition to end a sentence with.

So why do I keep hearing people end sentences with "at"? I've probably heard it half a dozen times in the last few days. Next person who says it, I'm throwing the book at them.