In my writing classes, I award extra credit points when students find errors in any of my work (presentations, assignments, email messages, etc). I figure it (1) helps them better swallow the critiques I make of their work and (2) gives them an incentive to pay closer attention to details in writing.
It's especially fun and satisfying when someone points out a mistake I've made in the middle of class.
Usually students find proofreading level errors, but today, a student found an error of some substance. I had inadvertently credited Shakespeare's "All the world loves a lover" to Emerson, who actually said "All mankind loves a lover." Technically, I had borrowed the syllogism the quote was in from someone else. A good reminder not to trust sources to have everything right.
Well played, attentive student with an iPhone and access to a database of quotes. Well played. I served; you won the point.