We test fired 12c first, and witnessed a very odd series of events:

After the smoke cleared, we stood there scratching our heads. We had watched the engine explode, but there were no pieces of it to be found.

Suddenly, the engine casing fell out of a sky and hit the ground with a thunk. Later, going through the video frame by frame, we realized that the plug had come loose, and the engine had backfired, literally, launching it up and out of our rig. In fact, you can see it in the third picture, the single frame where it is out of the rig, before it was propelled upwards. We believe that the "explosion" was caused by the fuel, which was left behind by the rocket, and was now burning on all sides instead of just one. This caused a huge cloud of smoke, similar to how the fuel had burned when left on a plate.