Originally Posted by
mcoomer
Dave,
To be clear, are you talking about one of the small jockey wheels on the rear derailleur or a cog on the cassette? If you're talking about the lower jockey wheel on the derailleur check that you're derailleur cages isn't bent. I can't really think of what else might knock the chain off a pulley. If you're talking about a cog then it's a good chance that your rear derailleur need to have the limit screw adjusted.
Post up a clarification please.
Mike
+1
As above, hard to tell exactly what you are asking about, but a chain is most likely to fall off a derailleur pulley because the derailleur or deralleur hanger is bent. Most bikes have replaceable hangers that are designed to bend long before the frame or derailleur. If the bike was in a crash (not even too serious) or stored in the trunk of a car (Esp. under a case of beer) or left outside somewhere it was likely to be kicked (outside of a bar overnight for instance) then this is very likely.