The Hollywood Reporter offers a few video clips of "failed" jokes at past Oscar ceremonies, leading off with David Letterman's famed "Oprah/Uma" joke that as you'll note in the clip there, didn't totally bomb the way some remember.
A friend of mine who worked on the Academy Awards that year told me the following: Letterman actually had a longer joke planned there that would have involved going through the audience and introducing a number of stars with odd first names to one another. The problem with it was that it would have involved getting cameras positioned to get shots of those stars…and Dave only decided shortly before airtime that he wanted to insert that joke into his monologue. When the director was informed of it, he said (approximately), "We can't do it. Right after his monologue, we're presenting Best Supporting Actor [or maybe it was Best Supporting Actress] and I need my cameras placed to get shots of the nominees. We mapped this out days ago and it's too late for me to rearrange everything to get shots of nine other people just before that."
Told that what he wanted couldn't be arranged at the last second, Dave instead opted to just do the first part of the joke, which was probably a mistake. The whole thing might have been a lot funnier. In any case, so what? It was just one joke that didn't get as large a laugh as someone hoped. You hear those in every monologue and I sure don't think it was the embarrassment that some made it out to be.
Within the Academy hierarchy though, they weren't fond of Letterman as host and the Oprah/Uma joke was a symbol of the problem. The complaint was along the lines of, "Dave didn't understand or care that this wasn't supposed to be The David Letterman Show with occasional interruptions to present some silly awards. The man only knows one way to do a TV show and he kept expecting everything to be done that way." My feeling was that it was a slightly-unfair criticism. You ask Dave Letterman to host your show, you shouldn't moan when he comes in and acts like Dave Letterman. I thought he was the wrong guy for the job but he did just what they should have expected and he was more entertaining than some Oscar hosts…like, say, most of them.