The joke that Bender tells but never finishes (while crawling through the ceiling) actually has no punchline. According to Judd Nelson, he ad-libbed the line. Originally, he was supposed to tell a joke that would end when he came back into the library and said, “Forgot my pencil”, but no one could come up with a joke for that punchline.
I know you want it to stay pleasant around here, but there are so many things that are so much better. Like silly, or sexy, or dangerous, or brief. And every one of those things is in you all the time, if you just have the guts to look for them.