Minecraft wiki entry for Historical_effects, associated text: Every power of 2 that the player goes, it becomes harder and harder to walk along the axis the player is traveling on.
At X/Z ±4,194,304, the player falls through the blocks. However, if the player teleports beyond this point they can still stand on the blocks but cannot move along that axis.
At X/Z ±8,388,608, the player can no longer stand on the blocks and falls into the void.