Related to headdesk and facepalm but only pertains to when using a computer.
...unless you're weird and carry along a keyboard with you, but that's your prerogative.
Ignorant Person: Hey I'm watching this awesome movie. It's called 10 Things I Hate About You.
You: I love that movie. And Heath Ledger.
Stupid Person: What are you talking about?? Heath Ledger is NOT in this movie. You're stupid, I just checked the info guide and he isn't one of the top characters!!