Yes, I am never again going to remove a derailleur myself. though I do believe this could happen to anybody. I have never cross threaded a bolt on a bike before, but this one is just much harder to judge visually. It would be nice to have some sort of guide piece that keeps the RD perpendicular to the hole before threading.

To answer questions that came up: Finger threading is of course impossilbe; I used a hex wrench (standard with a 90 degree bend in it. I stopped turning as soon as I got resistance and forced nothing. Yet at the end after may be a dozen attempts I saw the thread damaged (not destroyed all the way through). Interestingly the bolt appears to be utterly undamaged, at least to the naked eye. Having a bolt that much harder than the thread is of course a recipe for disaster. The hanger is part of the frame, not detachable. Otherwise the problem would be smaller I guess.

This thread is an awful lot easier to cross thread than the others on the bike. People giving advice to amateurs really ought to put in a warning.