This is a similar question to What exactly happened to my rear derailleur? but with some significant differences.
My bike had recently been serviced (about 3 weeks/500 km previously). All that had been done to it since that was a little tightening of new brake cables and oiling the chain the day before a planned big ride. The RD seemed fine when I oiled the chain. Over the first 100ish km of the ride (link includes pictures of the aftermath) I'd used the full range of gears, front and rear. On a gentle uphill, at least a few seconds after changing down, there was a nasty noise and the back wheel locked up. I barely unclipped in time and got off to find the RD tangled in the spokes and wedged against the seat stay. After two failed attempts to set up and emergency singlespeed conversion (snapped chain) I got the train and a lift home. No spokes broke at the time but several are damaged (two snapped after about another 150 km with a borrowed rear mech).
All parts were present including the B screw and both cage/axle screws (though one was torn out of its thread) and the mounting screw was still firmly in the hanger (which despite being solid steel was quite bent).
I've started putting the bike back together, but what should I be looking out for either as I reassemble it, or before a ride - I've got a tour coming up.