I was playing with my brother until his name started showing halfway between his body (in Minecraft obviously) then, because we were doing PVP, I tried attacking him but it wouldn't hit him. At first I just thought it was a skill issue but after I was sure I was hitting him. I hit his feet, where hit name was, and he started taking the hits. Sorry, I have no images because this happened a week ago and I didn't know you could report bugs to Mojang.

tldr: a player's hitbox was halfway down their Minecraft skin.