Stripped bolt.
Been there, seen it, done it, bought the T-shirt, seen the film and got the collectable figurines.
One thing puzzles me though. In order to actually have a stripped bolt in the first place, some SOB must have stripped it. I wasn't aware that NASA had previously taken the Hubble into Kwik-Fit* for servicing.
*Note for Americans. Kwik-Fit is a franchised chain of bodgers^H^H^H^H^H^H^Htyre and exhaust fitters. Their favourite tool is the air hammer with the torque settings permanently fixed at "....and the horse you rode in on", invariably fitted with a socket that's half a size too large. Rumour has it that they invented the rounded nut.