All start with the following command :

summon MinecartRideable ~ ~5 ~ {Invulnerable:0,Riding:{id:Slime,Size:-10,Invulnerable:1,ActiveEffects:[{Amplifier:1,Ambient:1,Id:14,Duration:199999980}],Riding:{id:Slime,Size:2,Invulnerable:0,ActiveEffects:[{Amplifier:1,Id:14,Duration:199999980}],Riding:{id:Fireball,direction:[0.0,0.0,0.0]}}}}

As a mapmaker, this creates a good flying and controlable object you can get for hole new kinds of maps... (you can control it by left clicking in the direction you want to go)

However, this new things has pointed out many bugs concerning ridable entities :

• from time to time, if the object hit a wall, Minecraft will stop render anything but the background colour (black at night, and blue at daytime)
  (even options like escape will not appear and the only way to get out of this mess is to force quit the game, and come back, and only then, you will be able to see the respawn screen)
• In the same conditions, when you don't get this bug when hitting a wall, the fireball will explode (logical) but the minecart will move down, through all blocks, and you will die in the void...