In The Muppet Christmas Carol, in the scene where Belle breaks up with Scrooge, I misunderstood the meaning of her saying "another year before our wedding".
As a kid, I thought it meant "just one more year before our wedding", don't think I noticed that she didn't sound excited when saying that. Can't remember when I realized what she really meant was "we have to wait at least another year before our wedding, which we've had to do before".
During Fozzie and Rowlf's rendition of "I've Got Rhythm", it took a few years before I knew what was going on. I didn't understand Fozzie was singing bad, or why Rowlf kept basically saying he was wrong. When Rowlf showed him how to sing, I thought he was telling Fozzie what his line was (as opposed to how he's supposed to sing the line) and wondered why Fozzie apparently got it wrong when he repeated. I didn't realize that Rowlf changing it to "I Don't Got Rhythm" was because he didn't (of course when I first heard that, I didn't know what rhythm was, though unlike a lot of other things I was confused by, I actually did ask within a short amount of time after seeing it). It wasn't until I watched "It's the Muppets!: Meet the Muppets" with the closed captioning on, where the captions say "(out of rhythm)", that I finally knew.
Similarly, I had a bit of trouble understanding Statler and Waldorf's heckling in that song. After Rowlf and Fozzie sing the line "who could ask for anything more?", Statler says "we can!" and Waldorf says "yeah, earplugs!" I couldn't understand what Statler was saying, but Waldorf's line made me think that Statler said "Earplugs!" (even though I didn't think it sounded like that) and Waldorf was just repeating/agreeing.