This is the Best Placebo Ever!
So, this happened during a Pathfinder campaign me and a few of my friends had a while ago. Besides the DM, there was a Ranger (me), a Rogue, a Paladin, and a Sorcerer who loved trolling. During our quest, we ended up coming across this abandoned mansion that was booby-trapped like none other.
For the first fifteen or so minutes of gameplay, my character alone got bit twice in the leg by bear traps, had a poison dart shoot him in the arm, and got shot by a ray of evil from a seemingly haunted painting. Needless to say, it was going to be a very long quest if this continued.
Sorcerer (ooc): Alright, I’ve got an idea. (to DM) I cast Prestidigitation on [Ranger] to make it look like I put a barrier up around him.
Sorcerer (ooc): I then tell him that I’ve made him invincible as long as the barrier is around him.
Me (ooc): I don’t think Prestidigitation works that way.
Sorcerer (ooc): Maybe not, but your character doesn’t know that.
Me (ooc): Oh, you fetcher!
DM: Alright, [Sorcerer], roll to Bluff, [Ranger], roll to Sense Motive.
We both rolled, and I rolled a low enough Sense Motive and [Sorcerer] rolled high enough Bluff that my character believed his…at least to a point. So, we get to the end of the hall and notice a small divet in the floor. Our Rogue is able to find that there is a trap there, but she can’t tell what or where it is.
Me: Well, let’s do this. (ooc) I rush towards the door.
DM: Alright. (rolls, then pauses) As you run towards the door, two large axes swing down to cut you in half…
Me (both in and out of character): Frick!
DM: …but something in the trap mechanism gets caught, and so they both stop an inch away from you as you pass by, and then drop to the floor.
All of the players were in complete shock. Eventually, my Ranger turned to [Sorcerer].
Me: How long is this invincibility supposed to last?
DM (to [Sorcerer]): Roll to Bluff.
Sorcerer: (rolls Nat 20) Indefinitely.
Me: (turns to the stairs) I’M INVINCIBLE!!!
What happened from then on was incredible. Every trap my Ranger came across failed epicly. Flamethrowers from the walls? They sputtered out as I passed by. Haunted paintings? Their evil rays bounced off my armor. Spikes from the floor? They’d jam up as I’d pass by. The only trap that worked was a trapdoor, and even then I somehow only took one point of fall damage.
Me (ooc and after clearing the mansion): This is awesome!
DM (rubbing temples): This is a headache.
Sorcerer (ooc): It’s like a placebo, but better!
For the rest of the campaign, my Ranger genuinely believed he was invincible. While the DM didn’t have as bad of a Fail-rate as he did in the mansion from then on, my Ranger still managed to be the one who dealt the most damage and survived the most mortal wounds.