If the block at the leg area of the player changes to collide with the player's hit box, the player will freeze being unable to move or look around. Doesn't work on mobs.

this includes sand dropping onto the player and turning into a sand block at leg level. Also
standing up against a wall that is connected to 2 blocks on opposite sides at leg level and breaking one of the blocks to change the wall to that thicker disconnected state. Other similar situations are possible.

Edit: also happens with doors, fences, things that have collision boxes that can change.
re logging also doesnt fix it.

The only way I have found to be able to escape it is to break the colliding block (if possible) or go into spectator mode