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Pathfinder Battles Case Subscriber; Pathfinder Adventure Path, Modules, Roleplaying Game Subscriber
Crypt of the Everflame had some illusory monsters and had this to say:
These [redacted] are illusory. Whenever they hit or are hit by one of the player character, they react appropriately, but give that PC a DC xx Will save to notices that these [redacted] are not real. Characters who are told that the [redacted] are not real receive a +4 bonus on the save. If the save is failed, the character believes the wounds caused by the attacking [redacted]. Once all of the characters have disbelieved or all of the [redacted] are slain, both the [redacted] and the wounds caused by them disappear.
That was very early in PfRPG's life and might have been contradicted since then, but I always liked the mechanic.