On multiplayer, go into a private game with yourself and 1 other person (the other person has to want to help, but not actually do the glitch). Drive the wagon near a cliff (but don't drive it off!), then have your friend sit in the driver's seat. Stand right next to the driver's seat. Count to 3, and when you get to 3 both you and your friend have to start pressing Y repeatedly as fast as you can, until you fall onto the ground. Finally, walk off the cliff and you will be floating. If you try to walk up a mountain after doing the glitch, will walk through it and be underground. To get somewhere the normal way after doing the glitch, walk to a place where it looks like your feet are touching the ground. Then call your horse. You will be able to ride on the ground. Once you get off your horse, the glitch will work again. Just walk up an incline and when you reach the top you will just keep walking upwards. Walk off a cliff and walk in mid air.
Added by: DaReconGuy Aug 16th 2010, ID#6183