Long wall error refers to a bug which occurs when unusually long linedefs are present in a level. Such lines may appear to shift back and forward as the player moves relative to them.
The effect can only be seen on veritably long walls (greater than 1024 units) composed of a single linedef. Greater lengths accentuate the effect more. The problem can be avoided by breaking long walls into multiple linedefs.
The effect is caused by loss of precision in calculations internal to the rendering engine. Doom internally uses fixed-point arithmetic to perform calculations, for speed. As these fixed-point numbers have a limited range, incredibly long walls can cause an overflow, leading to the wall being rendered at the wrong location. In extreme cases (walls of greater than 10000+ units in length), collision detection stops, and things are able to pass right through a wall.