I think perhaps my favorite flubbed lyric ever is 7.15.1991 in Cavern. Going along through the song, it sounds like they are going to add a couple extra instrumental measures or jam a little between two of the verses. Until Trey asks something along the lines of: "Does anyone else remember the lyrics to this? Because I sure don't."

Its like a paraphrase of the old adage about pizza and sex: "Even when they're bad, they're still pretty funny."