Entering in tp X/Z -30000000/30000000 will display a message saying that the coordinates cannot be over 30000000 and will not work. The game will load X/Z -30000000/30000000 but will act as if there was a barrier in its place and will not let you walk there. This all applies to both the Overworld and the Nether and in the 1.9 snapshots, applies to the End.