Highest gear "jumps" under load When I'm in the highest (smallest) gear and pedaling hard, it sometimes "jumps".  I'm not sure if that's the right word, but I feel a bump in my feet for a moment and then I continue pedaling.  It feels like I'm shifting gears, but I'm not.  I've spent some time today adjusting the rear derailleur, and all the other gears are good.  Could this be caused by a worn out sprocket?  I don't use this gear all that often, so I'm not sure why it would wear out faster than the others.  Pedaling moderately is fine, this only happens when I push hard.

This is a 9-speed Shimano Sora.
It wears fast because there are fewer teeth in contact with the chain, but that might not be the cause. Here are some other possibilities: 


A stiff chain link is most noticeable with a small sprocket. It's also more annoying if not more noticeable when under load. This tends to cause an even rhythm under steady pedalling, about one every 3 pedal strokes depending on your chain length and chain ring. 
The high limit screw needs a tweak for some reason and the chain is trying to change gear. Even the chain tension is a possibility. 
The derailleur has taken a slight knock and isn't quite straight. 


The first two of these are best checked on a stand, turning the pedals by hand and watching the rear mech.