All versions of the Doom engine, including the variants used for Heretic, Hexen, and Strife, have exhibited limitations with very tall areas. This sometimes simply manifests as rendering glitches (such as the screen filling up with a horizon-like flat), but it can also reliably crash the game. Although not commonly known, these limitations can begin to manifest with as little as 2500 units' difference between floor and ceiling. It should be noted that this phenomenon is related to but is distinct from Moiré error, which was restricted to visual effects and occurred at much smaller height differences. That problem was repaired in version 1.4 of Doom, but the greater issue remained.