Flying enemies stick to other monsters and players

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In vanilla Heretic and Hexen, flying monsters may encroach into other monsters' or players' hitboxes when moving vertically, causing both monsters to become stuck until one of them is killed. This occurs most frequently in Heretic maps where fire gargoyles are mixed with regular gargoyles or disciples, as they are missing from a hardcoded check intended to prevent flying monsters from moving over or under each other. However, it can more rarely occur with other combinations of creatures in both games, especially when they are near walls.

Another instance where this can easily occur is when the player is moving toward a flying enemy and it begins to "autolevel," a feature of flying monster motion in the Doom engine which causes such monsters to try to stay in the same general range of Z coordinates as the target they are attacking. There are several points in the code, particularly in the function P_Move, where the game fails to check for overlap between the hitboxes which would be necessary to prevent this problem. This is common when near opening doors with a flying enemy behind them, since the door being in the way prompts the enemy to fly downward immediately, and can also occur with greater frequency when the player is spanning across a change in floor heights.

When this problem occurs with the player, it will often be necessary to load a save in order to continue the game, as with gargoyles, the player will be killed by their scratch attack but cannot successfully retaliate with most weaponry. With disciples, the player may become soft-locked, with neither entity able to damage the other.