I wanted an 8-cog 12-32 cassette, which you can't find any more. Sheldon Brown said, "There is no problem mixing 7-speed or 8-speed flat sprockets into a 9-speed cassette, or vice versa." So I got a new 8-speed Sunrace 11-32, and started looking for sources of cogs to swap in. Found suitable Shimano clusters (an 8-speed for the 12T with its integral spacer, and a 9-speed for the 14T & 16T) in a used parts bin - but looking pretty new. Got the hybrid cassette together, and tried it out. The shift from 18T to 16T - across the boundary - just didn't work. The derailleur balked, and then jumped two cogs on the next shift.

So I tried taking all the cogs, except the 12T, from the 9-speed donor (using 8-speed spacers). And that worked. The puzzle then is that both source sets are viable and shift smoothly - but the mix doesn't work at all.

9-speed cogs are only .02 mm thinner than 8-speed, adding up to less than 1/100 of an inch for the whole set; pretty sure that, as Sheldon says, that's not an issue. For lack of any other possibility I can see, my conclusion is that the Hyperglide ramps are more important than Sheldon says, and the two brands don't synchronize them. If so, worth knowing for the would-be cassette mixer. Thoughts?