Originally Posted by
Deontologist
.... There is no way the bike has seen more than 1000 miles. I'd be willing to bet less than 500 miles. And here I am with a worn out chain and cassette. What gives?
I can't say why it wore out so fast, or even if it did. It's always possible that the chain had a few thousand miles on it before you came along.
In any case the best predictor of cassette wear is the stretch of the prior chain. Did you measure it?
In any case, over the net diagnostics aren't super reliable, so save the old cassette in case the new one skips, in which case you'll be looking for another cause.