I've noticed when I am doing stuff such as gear indexing (and my bike is up on the stand) that there is a slight wobble or oscillation to the smallest two cogs in the cassette. If you look carefully when the wheel is turning at speed, there just seems to be an ever-so-gentle movement. It is quite soothing to watch, like being rocked in a cradle :).
Full confession: I recently "expanded" my bike service skills by my servicing my wheel hubs. I followed such sources as Leonard Zinn and Park Tool videos very carefully. I made sure of a little play in the hub between the cone and lock nut that would be taken up by the quick release lever's closing. I also torqued the cassette's lock nut to the recommended 40 Nm.
Before I serviced the hubs, I had never noticed this subtle cassette wobble, which is not to say that it was not there before. The smallest two cogs in my 8-Speed cassette are separate, individual cogs from the other six, which are in a cluster, so perhaps this accounts for slight movement, even with a correctly torqued lock nut? As far as I can tell the axle is straight and the rear wheel is adequately true. The bike rides like a good 'un, and I do not feel remotely unsafe and there are no riding or handling issues.
Is this just what road bikes do, or did I mess something up on the hub service? If so, what remedial action might be necessary?
P.S. I posted a very similar topic a day or so ago, but can see absolutely no sign of it. Profound apologies if you replied to that one, but I honestly think it did not save. I cannot see it. I am not deliberately wasting forum members' time by posting twice :).