Originally Posted by
FBinNY
...The mechanism that causes skipping with new chain/old sprocket is tricky and subtle. It's not that the chain slips under load. It doesn't, which is why, this type of skipping is unheard of in single speed bikes. The problem is with all the available slack in the lower loop, it's easy for a chain to snag and ride up onto the teeth, then when that unengaged bit of chain comes around to the top you lip forward a bit...
I see it differently. The teeth on modern cassettes barely come to the 1/2 way point of the roller to facilitate shifting whereas single speed cogs have longer teeth. Just a slight amount of wear at the top rear edge of the shorter teeth can cause a new chain to jump. This occurs with a new properly adjusted derailleur and with the big ring tensioning the lower run so there's virtually no chance of the chain not feeding onto the cog properly. You can observe it from behind on a trainer and the chain looks perfectly aligned with the bottom of the cog when it happens.