Join Date: Jul 2006
Location: Sammamish, WA
Bikes: Cannondale Prophet, Specialized S-Works SL2, Specialized S-Works Stumpjumper
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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.