OT another officiating trivia brainteaser

[rotationslim]

Another OT Trivia Teaser, not about basketball, but about officiating:

Name the sport where a non-participant routinely throws a projectile at the referee during the contest. The projectile is not a ball or any other implement used by participants in the contest.

This is an Olympic Sport, although this quirk in the way the sport is played is not necessarily exhibited at the Olympic level.