I suppose there is a mistake in Pawn::LookForMonsters() function. In FOREACH loop there is no condition for 'mo' = null (I believe it can be null in some cases). I tried to make a monster that acts as an ally for player and he uses LookForMonsters() in his overriden function A_Look(), and when I place him on map and launch it, game crashed; until I put to LookForMonsters in FOREACH loop additional condition:

if ( !mo ) continue;

and then it worked.

It can't be null. Look at the FOREACH definition in common/utils.vc.

Huh, seems then that I mistaken. Or it was something depending on machine. First I tested it on ThinkPad, then it didn't worked, but when I tested it on PC it did.