Here's a prank that's also an interesting experiment in social psychology. In the middle of a busy public square, a big sign over a red button says, "DO NOT PUSH THIS BUTTON." Of course, random people walking by inevitably do push the button. At which point, everyone in the square appears to drop dead. So what does the person who pushed the button do? Does he/she try to help the people? No. Every single person who pushed the button runs away, as if trying to escape being found out.
The prank was filmed in a square in Rio de Janeiro. The TV presenter Silvio Santos provides a narration (in portuguese). More info at forbes.com.