A couple of stories happened at the Dam Regatta at a reservoir in Columbus, Ohio.

Mary and I were on a Hobie 18 and there almost no wind (pretty typical) and we stayed right on the line. With about a minute to go a fisherman buzzed up and sat there right in the middle of the line.
The RC was yelling at him to get out of the way and he stubbornly replied, "This is MY fishing hole!"

The next year Tom Tannert's son was leading a Hobie 18 race, rounded the leeward mark and headed up wind. Somehow he caught a fisherman line and broke it (nothing lost, just a lure.., or perhaps only a hook and a worm)
The next lap around the guy jumped in the water and grabbed his rudder and would not let go.

They do seem to mark to a different drum. <img src="http://www.catsailor.com/forums/images/graemlins/tongue.gif" alt="" />
Rick