Shimano's HyperGlide-C (11-speed), are the hubs backwards compatible?
Can I put an 8/9/10-speed cassette on a Hyperglide-C hub?
Can I put a Hyperglide-C cassette on a Hyperglide hub?
Sheldon Brown to the rescue: "Shimano uses the trademark "Hyperglide-C" to designate a system with an 11 tooth sprocket. The "C" stands for "compact". These systems are used with smaller-than-usual chainwheel sizes, or on bicycles that have a small drive wheel, or to achieve higher gears.
Due to clearance problems, the cutaway between the splines on 11-tooth sprockets only goes halfway through the sprocket. The matching splines on Hyperglide-C bodies don't go all the way to the outer end of the body."
To summarize: Hyperglide-C cassette on non-hyperglide C body gives wobbles (which you can correct with a 1 mm spacer).
In the reverse case, you can remove some spacers from your cassette (if you have them) to get it to fit (though I'm not really sure when this case would practically occur).