When you spawn an Minecart with an fallingsandenity above an commandblock, the commandblock disappear! You can't walk trough it and it will glitch when you try to jump on it or walk trough it! You can interact with other blocks trough the hidden commandblock. When you place an block at the location of the hidden commandblock, the commandblock will come back but the command in it got removed! Here is an video I made about this bug: http://youtu.be/y9pRr9ziN2c
This is the command that I put in the commandblock: /summon Minecart ~ ~ ~ {Riding:

}