It’s just way too easy to mess with drunk people.
Once when I went to a formal dance, I invited Jan to come along. We were wearing reasonably identical tuxedos and the biggest difference was that I had a blue bow-tie and handkerchief while Jan’s were red. It was great fun already, and then someone starts the twin questions; we just answer and have fun.
Much later in the evening, we meet one of these people on a staircase. He’s gotten pretty drunk in the meantime. He stops us and chats with us, and we convince him that he’s seeing double and that he’s only talking to one person who isn’t even a twin.
We overheard him telling this to someone else afterward. It sounded like he believed it.