This happened on my 1965 Falcon yesterday.
I replaced the rear wheel cylinders with new ones, the brake shoes with best-I-could-buy Bendix, all new hardware(springs retainers etc.) turned and trued FOMOCO NOS drums. System completely flushed and fresh brake fluid installed. System bled and brakes adjusted, test drive and final tweaking completed about 10 pm last evening.
So six am this morning I am in the 300E about to leave for work. I look over in the next stall and the Falcon sitting there shiney and ready to go. O.K. I know it has good brakes now and needs to be driven so I swap cars.
Falcon is started and warming under fast idle (this is a heavily cammed motor) and garage door opened. Check the rearview and see nothing but darkness. Shift to reverse and back briskly about six feet into the front bumper of my son's pickup truck!!!!
OMG!! I DID NOT DO THAT!?!?!?!?!
Oh well, bent bumpers and pride mangled, but stuff happens, right?