I duuno about come right off, but some plonker I know had the sprocket trepan a hole in the grey plastic cover
So you need to keep an eye on the sprocket reytaining plate, and make sure it's not distressed. The grunty 2V engines are particularly prone to this (900SS, ST2).
There was, a while back, a new retaining plate introduced. The old ones wore the teeth away quite quickly. Look identical bar the new one is finished of in black rather than being uncoated metal. Also make sure you loctite the screws when refitting the plate.
Mood: #65 Double Desmo Due Champion - Class B 2007, Class A 2009
Now thats why the bolts were so hard to remove when I geared her down, and yes I did loctite the bolts back, heard this happened before on other bikes...