Since I was the last one to install that particular caliper, I believe I have a coherent explanation for the problem. When I first looked at the car to put the x-drilled rotors and HawkHPS pads on there I noticed that one caliper had become completely seized and the other was fast on the way. Since I HAD to complete all the work in a day, we had little option but to go get a pair of junkyard calipers, no where had stock S14 rears in stock. While they looked fairly old the piston retracted all the way and the install wasn't really a problem. I re-used the bolts from the previous caliper, which I believe may have been damaged from the previous seizing (undue stress on the sliding pins, etc).
To be exact, the outer half of the caliper came off, not the one that is bolted to the spindle, the part that the lines connect to. I believe the bolts for the sliding pins were stripped after several hard stops the threads were pulled clean off, ala brake caliper explosion. I've seen it happen before, but that was with a seized caliper. I'd have much rather put on new calipers and bolts from Nissan, but the situation just didn't allow it. The good news is that at most it should only cost a new loaded brake caliper, possibly a line, and one rotor. I would however, suggest replacing the caliper on the other side, if not at least the bolts and mounting hardware.
While a real simple explanation could be that I left the bolts loose, I HIGHLY doubt that as I am VERY positive I gave them the twice over, and they certainly wouldn't have lasted for weeks if I'd left them loose.
As others have said, at least it's not engine related. Suspension and brake problems are easy.....if not somewhat frightening. I'm glad our most valued female member (and her car) lived through the occassion

.