"Your, You're" is in the same family as "Their, they're, and there." When someone uses the wrong one, it's very fun to make fun of them, but it's also incredibly easy to make the mistake yourself. Sometimes I'll have been writing an article for a couple of hours and things slip, or I'll be in a heated Facebook troll session and use the wrong one. We'll all get caught using the wrong one someday, but how this went down is especially cathartic.
While on a tirade against people who get "their, they're, and there" wrong, this Facebook grammar nazi made the crucial error of using the wrong "your/you're." If you want to immediately look foolish, that's exactly what you do. Everyone knows you can't talk smack about something that you're doing in the exact same moment you're talking smack.