Effect: Your friend beats his head on the bar repeatedly, to everyone's delight.
Secret: Careful with how far you take this one. It could get nasty. Explain to your friend that there's a peculiar attribute of physics that makes a coin rather difficult to remove from your forehead. Demonstrate this for him by taking a quarter and using your thumb to press and stick it to your forehead. Now tilt your head to the right and lightly bang down on the bar. "See," you say, "the force of the banging is all downward, so it takes two or three good whacks to get the coin off." And sure enough, after a couple more whacks, the coin falls off your head.
Here's where it gets good: If you've sold the novelty well enough, your friend will want to try it out. Take the quarter and push it for 10 seconds or so on his forehead, but make sure your fingernails are wrapped around the edges of the coin. This will allow you to take off the coin as you move your hand away. If you pressed hard, your friend shouldn't be able to tell that the coin isn't on his head.
Now, have your friend do just like you did, and bang his head on the bar until his coin falls off. The game ends when he hurts himself or figures out there's no coin on his head, whichever comes first.