I've never experienced that, but I can imagine the havoc it would cause.
Imagine; your magnet(s) vibrate loose from the flywheel while running. If the magnets are internal to the flywheel, centrifugal force will keep them in place until the engine gets shut off. At that point, I can see one of them attempting to stick to the coil's iron core, and the next time you try to start it, it gets completely out of position and starts tearing stuff up inside the flywheel while you're pulling on the starter cord, or using the electric start.
If the magnets are externally mounted, then they'll attempt to fly out and immediately bust stuff up. I can imagine the coil would be the first casualty.
Just my thoughts.....Roger