Wall lag is a glitch in the game and a natural phenomena that occurs usually after around 3500 distance. The player
Wall Glitch

A player gets a view of caves and Hell while in a wall glitch.

will begin to lag through solid blocks like they aren't even there. Many times, this lag gets worse the further you go. At around 8000, you will be able to walk through all blocks that don't go above your head. This poses a substantial problem in caves and when building houses. As of now, there is no known way to avoid this. At about 12,000 the glitch seems to start to slowly fix itself.
Block Glitch

This player is actually standing inside of a non-solid block.