So, at the beginning of the module the players fight orcs who turn out to be an illusion, and once they kill the orcs or they all succede at the save to disbelieve their wounds all disappear and they are back at full hp. That said, I am wondering what happens if a PC is killed by one of the fake orcs. Do they die for real? Or do they just "die" until the orcs are killed/disbelieved by everyone else and then "wake up" with full health again? I would guess the second one based on how the wounds work. I would rule that Deathwatch or the like would reveal that the "dead" person was really alive though, and would likely clue them in that the whole thing is fake.
I want to say that very thing happened when we first ran the module many years ago. One of the orcs got a solid swing in and dropped the PC who fell down unconscious from the shock of the illusionary blow. The other PCs roused him to his feet where he realized he was miraculously unharmed, much to his own chagrin at having passed out.