meaculpa

This past summer I had to replace the rear cassette & chain on my commuter. 500+ miles later, the teeth are worn & the chain is slipping...again. I wonder if its my constant shifting thats to blame. I have a habit doing this I think because of being protective of my knees. But my lack of skill makes frequent shifting harsher on the drivetrain.
The cog was Sram I think.

z415

Depending on riding style (i.e. shifting), weather, level of components and a whole bunch of other things, around 500 miles is not too bad for the set.

I would add that you should make sure it shifts very well (read 'perfect') to prolong life. Having the whole set up a *little* off can shorten life due to premature wear.

sch

500mi is lousy mileage for either cassette or chain. My experience,
not commuting or off road and not being compulsive about clean
or lube, is that a chain will last 3000-6000mi, a cassette
will last two chains, and chainwheels (the most used ones anyway)
will last 3 chains. Others get better or worse, but 500 is way low.
Riding a very gritty chain in all weather conditions
will certainly accelerate wear and use up the cassette faster.
My longest lived setup was the 8spd front drive on my bent in
which the chain, cassette and
CW all lasted 16,000 mi. Chain elongation was about 3/32
at change out and it was a KMC chain.
Shifting has very little to do with cassette wear, and brifter users
in general shift 3-4x as much as downtube shifters do, with no
difference in wear rates in my experience.

Bob Dopolina 

Did you happen to ride near a beach shortly after installing the new chain and cassette? Beach sand is sandpaper and will promote this kind of wear.

HillRider

I was wondering the same thing. Only sand or something similar could cause that kind of very fast and abnormal wear.

My experience agrees with sch.

Al1943

I don't think you can wear a cassette enough in 500 miles to cause chain slip.
If your shift cable needs adjusting the chain can off and on a cog or autoshift to another cog causing apparent chain slip. Any decent cassette should last 10,000 miles or more if the chain is replaced before excessive elongation occurs.

operator

500 miles would be enough if he's crosschaining big time, lives near an ocean or rides in the winter on one or a few gears exclusively.

Coyote2

Fixed it for you.

In other words, it'd be almost impossible to wear out a chain and cassette in 500 miles. His drivetrain (i.e., rear der and perhaps front too) needs adjustment, cleaning and lubing.