OPERATION HAND IS A GO.
Vicki Vale grabs the Joker's hand only for it to detach. Batman eventually saves her, meaning that OPERATION: HAND needs to go back to the drawing board with the note "Needs less Batman," but wait. When did he put that hand on? He didn't have a fake hand when he was throwing puny body shots against Batman's rubber abs. And he definitely didn't have a fake hand when he grabbed both of their wrists to yank them down. So, in the brief time that Batman and Vicki were hanging there, the Joker attached a plastic hand right in front of them. That's some amateurish supervillain-ing, and some much more amateur-ish Batman-ing for the World's Greatest Detective to see his arch foe slip a false hand on and just not say anything when the love of his life grabs onto that shit, but it also brings up a much bigger issue. The Joker has kept a fake hand around for that, and only that very specific moment.
That's fucking incredible and it shows that Joker is primarily concerned with accomplishing the dumbest tricks possible. And before you say "Oh, he might have had it around for another gag," remember that this Joker already created a joy buzzer that roasts you alive when you shake his hand. He's out to kill. He's not going to leave it in some other mob boss' grip and walk away. No, the Joker, before he even decided to throw his big, Prince-fueled poison gas event, decided that he needed to be ready just in case someone was hanging off the side of the very high place in front of him.