Ball joints

Depends on how fast you're going. Drums with clutch plates are rotating in the transmission. When you shift into reverse, a flexible metal band tightens around one of those drums to prevent it from rotating, then the gears inside it are forced to spin, and that causes a shaft to run backward. That's how it goes into reverse. When the car is not moving, the drums are all standing still. That is what is intended when the band tightens around its drum. It is not designed to bring the heavy mass of a spinning drum to a stop. It can do that for some time, but that banging puts a real lot of stress on it. Eventually the end of the band will crack off, then it will be unable to apply. The car will go forward when you shift into reverse, and if you shift into low. Or "1", the transmission won't stay in first gear. It will shift normally up to second gear.

The faster the car is going when shifting into reverse, the more stress will be placed on the band. The eventual failure could occur any time, even when shifting into reverse when the car is standing still.

If this was a one-time thing at a relatively low speed, say under 3 - 5 mph, it is nothing to worry about.