Found this out at 3:52 4/23/2013
Players can clip under the map, through walls/doors, and more, simply by using the Easy Precision tool on a veichle or seat, And getting in it as they glitch/clip through the ground/wall/etc.
Possible fixes: Restrict easy precision (which would REALLY suck, considering it is one of the most useful building tools availiable) or Restrict this tool from effecting any entities that aren't props. (not as easy, but preferable to restricting a useful gmod tool)