If you stand in a nether portal, and then sleep in a nearby bed before the portal teleports you, you will enter the nether at the EXACT coordinates that you were at in the overworld, instead of the coordinates scaled down by 8 (like you normally would). This allows the player to teleport thousands of blocks away from spawn instantly, depending on where they perform this glitch.

Similar to an old bug, where eating food in a portal would result in the same thing.