well as always there is a compromise. a leaner burning engine will run hotter, and combustion temperatures are also greatly increased. end result if too lean you get detonation and if bad enough bye bye pistons and more.
if it too rich the engine will run like crap, could be hard to start when cold outside. if excessively rich then the gas will wash the oil off the cylinder walls. this causes lack of lubrication, and if bad enough could seize the piston and cyl together. or just accelerate wear on rings and walls. also if its bad enough its possible to get a buildup of fuel in your crankcase which does as mentioned above to other parts. it could also be a small bomb(just thought of a funny/scary story).
anyways its much more likely to cause damage with the first scenerio, its about 10 times more likely. to get the engine to run rich enough to cause the problems in the second scenerio, you would have to be really stupid, and you should be banned from touching a bike ever again. lol.
it is possible, however, that an injector could get stuck open and flood the engine this way. if you see this happen immideatly shut off the engine and fix the problem. open the oil resivior and smell for gas. if you smell gas you need to flush the oil, filter too. when you go to start it after the flush, disconnect the fuel so the engine will not start and crank it a little to get some fresh oil on the cylinder walls to avoid a hot start.
damn sorry for writing so much... i get carried away