Chain rides up on sprocket teeth

[coffent]

I removed and replaced the cassette on my bike (making no changes).  Now I find that when the wheel is under load the chain climbs up on the teeth of the smallest sprocket (see photo).
Everything works fine with the other gears or when the wheel spins freely.  This seems to be the *opposite* effect I'd expect if the chain had stretched or the gear teeth were worn.  Instead, the pitch of the chain seems too short, not too long.  Can anyone explain what's happening and how I can fix it?

[Painkiller]

something does not look right about the high gear on that cluster. I would mention it is best to replace the chain and cluster at the same time. but your problem is typical of a worn chain or cluster or both.

[nfmisso]

Please post some more pictures, at least one from a bit further back.

To me, it MAY be that the chain is too long for the RD to completely take up; it looks like you have the chain on the smallest chainring in addition to the smallest cog at the back - not recommended.

[coffent]

Thanks for your suggestions.  I check the chain and it is not worn; also, the problem occurs with the chainwheel in either gear.  I've done some more investigation in the meantime and have concluded that the sprocket is indeed worn.  In case it helps anyone else, I found the Sheldon Brown article on "Chain and Sprocket Wear" quite helpful, as always.  With the wheel off the bike, I draped the chain over the smallest sprocket and pulled downward on the right side of the chain.  When I flipped the first engaged chain link on the left free of its tooth with my finger, the chain jumped forward just slightly.  This means the first tooth was doing all the holding, which, as Sheldon Brown says, is symptomatic of a worn sprocket.  Time to replace the cassette, I'm afraid.

[nfmisso]

Fortunately, cassettes are inexpensive, and you now have an opportunity try a different one.