When you are at a sporting event and the guy behind you loudly offers a play by play of the game, presumably to educate his kids. The problem is, he doesn't know the game well, so is mis-informing his kids and everyone around him. You grit your teeth, but can't correct him, as his kids will then know what he really is.
At a recent Giant's game the batter hit a soft fly that landed just over the first baseman's head. The Dad behind me called told his kids it was a can-o-corn, when it was really a duck-snort. What a Dumb-Ball!