I heard that when you ride a saddled pig sitting in a minecart, you can drive it with the keyboard, but the controls are backwards. When I went in F5 view, my head was backwards.
I'm pretty sure it's because normally if you spin your head whilst on only a pig, you'll steer it if you a have a Carrot on a Stick, or else, look around you. Since when I tried it the pig wouldn't move, so it would just do the moving animation every once in a while. Going back to the point, since the 'vehicle' is stationary, the pig won't rotate, therefore, your head does instead.

How to make this weird bug occour:
1.Place any block
2.Place a rail and a minecart on top of it. Do not push the minecart.
3.Build a pillar right next to it, having it one block higher than the minecart's postion (In most cases, the pillar's height is three blocks)
4.Spawn a pig on the side of the pillar so it falls in the minecart.
5.Attach a saddle to the pig.
6.Break the block below the minecart, or the whole structure if you wish.
7.Ride the pig in F5 mode. If you move your 'camera' around a bit, you can twist your head around 360 degrees, but having it backwards is the main bug itself.

So, there you have it Dinnerbone. Or Jeb. Or whoever!