Click on the photograph to see tooth details more clearly.
On the right is a new CS-HG50-8 cassette with my favourite 13-, 14-, 15-, 17-, 19-, 21-, 23-, and 26-tooth sprockets. I sure hope Shimano never stops making this model.
On the left is an identical cassette with about 16,000 km behind it. I strictly replaced the chain as soon as 12 inches of chain became 12 and 1/16th as measured with a ruler.
I did this five times and was starting to think that under this regime the cassette might last until the teeth wore to narrow spikes and broke off. But no! The sixth new chain skipped on the 15T and 19T sprockets. The 19T sorted itself out after 100 km but the 15T (a staple of Longchamp) did not, so I thanked this cassette for its service and retired it.
Looking at the sprockets closely, I suppose the 15T, being small, does not need as much wear as the 19T to cause problems, because it looks in better shape at first glance.
Sorry for not giving the cassette a bath before the photo.