When I was growing up in Buffalo, I used to go watch the Sabres play hockey. They weren't very good then, but they sure had mean fans. When a particularly despised opponent (e.g. the Boston Bruins) would come to town, fans would take coins from their pockets, heat them up by holding them in their hands for a minute or two, and throw them into the rink. Since the players wore lots of pads, helmets, etc..., it wasn't likely that a coin was going to injure a player by impact, but that wasn't the point. The point was much more subtle -- a warm coin will sink into ice a bit, at which point it becomes a significant impediment to the progress of an ice skate. Sometimes it took 30 or 40 minutes to get the pennies out of the ice and Zamboni the surface.
Found by avva