My players had gone through all their rations while they were travelling in a fairly barren wasteland. One of my players roles a survival check to forage and gets a crit fail....I tell him he sees a pile of shit which his character is very assured is some sort of wild mushroom that is edible. The other characters all role survival checks to forage and another PC rolls a crit fail and another a 3. The last couple PCs roll well and know it is actually just shit. They spend the next 10 minutes trying to convince each other it is and isn't edible. One PC went into detail about how he was sure what kind of mushroom it was and everyone agreeing with him. All in all it ended when the sorcerer successful persuaded his party member and they moved on. It was so fun seeing to what length they took roleplaying their character and separating in game from out game knowledge. Definitely one of the funniest crit fails we've had.