Note: This is client side. This does not affect anything, and if on a multiplayer server, I THINK (I'm not exactly sure) that other players won't see it.
Whenever I ride something, an entity, what I used in the screenshots were invisible horses with NoAI set to 1 (Note that if they will have NoAI set to 0, the glitch won't happen, because the horse always looks where you are looking, when you're riding him. But it will happen for minecarts, pigs, boats, etc.), whenever I go to third person view (F5 mode), my head does not match my body, and the head and body won't follow where I'm looking. It will only be fixed if you are looking where the entity you're riding is looking as well. You might not understand what I'm talking about, and if you don't, just look at the screenshots.