A situation in which one thing is pretending to be another. From Lemony Snicket's A Series of Unfortunate Events, Book 6, The Ersatz Elevator, page 129
"Not only are there no ropes," Violet said. "There's no endlessly looped belt, push-button console, or electromagnetic braking system. I don't even see an enclosed platform."
"I knew it," Klaus said, in hushed excitement. " I knew the elevator was ersatz.