Monsters can see through walls

Due to buggy line of sight calculations, monsters in vanilla Heretic may sometimes erroneously regard a player on the other side of a solid wall as visible, causing them to wake and go into chase mode before it makes sense for them to do so. This bug was most likely inherited from the Doom codebase, but the line of sight algorithm was changed in later versions of Doom.

Designer workaround
The most egregious cases of the bug, where monsters activate from sectors that could not possibly see the player, can be worked around by giving the map a proper REJECT table. Therefore, it is advisable for map designers who aim for vanilla compatibility to ensure that a REJECT table is built during the nodes building process.

Any remaining problems after that may, with some concession of design, be alleviated by adjusting the angle the offending monsters are facing.