[Lazy]

int NUM_FOR_EDICT(edict_t *e)
{
 int  b;
 
 b = (byte *)e - (byte *)sv.edicts;
 b = b / pr_edict_size;
 
 if (b < 0 || b >= sv.num_edicts)
  SV_Error ("NUM_FOR_EDICT: bad pointer");
 return b;
}

Looks like the error is made when the entity index is below zero or greater than the amount of entities in the game.