If you place a minecart on the lower part of a minecart track you can place infinit amount of minecarts inside each other, it creates a very loud noise and the minecart can go forever even without tracks.
Linked issues
Comments 3
Not really, this bug makes it possible to place more than one minecart on one minecart track, here is a demonstration: http://www.youtube.com/watch?v=MdH4nMvPuSQ
Duplicate of MC-14.
Duplicate: MC-6841