I can answer that one, Blue.
My rear fender actually ripped the fasteners out of the frame and through the fender then the fender wrapped under the rear wheel. It jammed there which meant instead of rubber on the road I slid sideways on a steel sled. http://motorbicycling.com/showthread.php?t=48419
The best option is to remove the fenders completely or reinforce them in such a way that loosening and getting caught in the tire is impossible. I'll be installing a new fender on my chopper but I guarantee it will never come off again. I'm welding it on.