Originally Posted by Ray Dockrey
This is normal even for a cassette. I have yet to seen one not wobble when coasting.
Mine don't and I've never had one that does. I've worked on dozens of bikes and never seen enough wobble to cause chain skip unless something was seriously wrong.
Either the cassette's lock ring isn't tight, there is a spacer missing, the freehub body is damaged or the freehub body isn't tight on the hub shell.
What make and model is the hub?