One of the pistons that support the tailgate keeps popping off the ball joint, probably because it has rusted away (like everything else on the back of my Subie).

As the tailgate starts to fall, the end of the piston arm usually smashes into the plastic housing of the brake light. The metal rod either catches on that, cracking it a little more each time, or slides off & the tailgate comes crashing down since the piston on the other side can't support the whole door alone. God forbid your head gets in the way.