My party was once climbing a tower to meet the arcane deity living at the top. The tower was full of magic, traps, conjured creatures, all sorts of shit.
They get to the next floor.
> DM: "Okay, the next room opens to a giant bottomless pit, about 15 feet across. Above it, half-way across, hangs a rope, attached to a pole further up that extends the diameter of the room.
> Rogue: "Okay, I jump to tarzan across the gap while the fighter and paladin take off their armor to put it into their bags of holding so they can make the jump.
> Rolls. Passes.
> DM: "Okay, roll a fort save."
> Rogue: "Okay... ummm.. " Passes.
> DM: "You grab the rope. As you clench on, you feel a sharp pain shoot through your hands, but you manage to hold on, swing and jump over. As you land, you look at your hands, and they're dripping from dep lacerations in both your palms."
> "The rest of you see blood running down the rope. Roll spot."
> "You all see that, under the rope, lie many small barbs, cutting up the palms of anyone who grabs it unaware. The bard notices the blood is pooling in mid-air over the pit, under the rope."
> The Bard: "That's not magical at all! That's just evil!"
There was an invisible bridge underneath the rope all along, and they could just walk across.