Freehub body damage by loose cassette rings.
By loose I don't mean not tightened, I mean individual rings not riveted to a spider.
This is pretty common on aluminium freehub bodies and an artifact of Shimano's 1980s design with shallow splines that did not foresee the advent of the weight weenie and aluminium freehub bodies. Campag was a bit more fortunate since it only redesigned its freehub body a few years after Shimano and did foresee the problem and solved it with deeper splines. Shimano is stuck with the legacy and so is everyone else who copied Shimano's design.
The rings moving on the body is no problem per se but can become an issue once the grooves are so deep that the cassette cannot come off. Then you need two chainwhips to reverse the movement and then slide the rings off.
The treatment at first is to file off the little bumps caused by the shifting material. Usually the problem settles itself because the aluminium quickly work-hardens and an equilibrium point is reached. Super strong riders even work through the hardened material and the only option for them is a steel freehub body or Campag equipment. Shimano's latest freehubs use a new deeper spline and special cassettes to eliminate this problem.
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In this example I filed off the really bad ridges, which as you can see, were caused by the loose rings at the cassette's small end.