This came up in a game involving Haunt 23, Tentacled Horror. (Spoilers will be kept to a minimum.) There are multiple different types of monsters in that Haunt: some of them have a Sanity trait, and others don't. One of the Heroes had the Ring, which let him attack with Sanity instead of Might. He attacked a monster that had a Might trait, but no Sanity trait, using the Ring. At the time, I told him that the monster had no Sanity trait and that he'd need to try a different type of attack, and he ended up doing a Might attack using the Sacrificial Dagger instead.
Was this the right thing to do, or should the initial attack have failed entirely? Or should we have explained which traits the monster does and doesn't have the instant the monster went on the map? The rules just say "you can't use a trait to attack against a monster that doesn't have that trait," but it's unclear on how to handle the attempt. This is particularly vexing, because all monsters have hidden traits and this is going to come up again.
(I looked at the updated Secrets of Survival and Traitor's Tome, and it turns out that the specific Haunt I was looking at no longer has that problem, but I'm sure this question will still apply to other monsters.)