This article or section is a stub. Please help the Doom Wiki by adding to it.

The "Anywhere Moo Bug" is a mathematical error in a tracer-based algorithm, which used the BLOCKMAP to do monster Line-of-sight (LOS) checks. This bug caused monsters to occasionally see the player through an arbitrary number of walls in Doom v1.2, and persisted into Heretic, Hexen, and some versions of ZDoom. It was eventually "fixed" by swapping the original algorithm with a BSP-based one. However, the performance penalty necessitated the creation of REJECT tables to speed up LOS checks, which brought along its own errors, due to the blind spots in Doom's REJECT tables.