Ragdoll blending
The transition between a physics-driven ragdoll and regular animation — usually in the hard direction, when a knocked-down character has to get back up.
Dropping a character into ragdoll is easy: swap the animation for physics and let the body fall. Getting it back is the hard part. The ragdoll ends up in an arbitrary pose at an arbitrary spot, while a get-up animation always starts from one specific pose. Switch over directly and the whole body snaps in a single frame.
The standard trick is to work out whether the character landed face up or face down, pick the matching get-up clip, move the character's root to wherever the pelvis ended up, and then spend a few tenths of a second interpolating every bone from the ragdoll pose to the first frame of the animation. That differs from an active ragdoll, which stays simulated with motorised joints throughout; here you leave physics entirely. Done carelessly, you get a dead soldier sliding up out of the floor like a marionette.