I was adjusting the indexing on my rear derailleur and couldn't seem to get the chain to work smoothly. From the back it looked slightly off from vertical, so I checked the derailleur hanger but it's fine. Then I noticed that when I tighten the bolt that attaches the derailleur to the hanger, if I tighten it all the way, the derailleur ends up not vertical. If I back off the bolt, taking the derailleur out, the derailleur wobbles relative to vertical. I can get good indexing if I back the derailleur out a couple of turns and eyeball it to be vertical.
This wobble with bolt turning during installation also occurs with the same model derailleur on a brand new frame. Is this by design? Do I need to worry about my derailleur flying off since it is not tightened all the way down?