No, the muppet line definitely isn't mine, but it's cute! :) I had that experience of thinking I was stealing from another author very sharply with one scene in "The Ice Age," to the degree that I asked about it on LJ before posting the story, and at de_nugis's suggestion actually took it to a story-finding board, where several people tried to guess what novel the scene might have been from, and no one was able to identify the source. I finally just put the scene in, in modified form. If it really was from a novel, no one who's read the story has ever noticed, and the image is pretty distinctive.

I think it's common for writers to worry their ideas come from a source outside themselves, even when they don't. Paul McCartney was apparently convinced when he first conceived "Yesterday" that he'd stolen it, and went around playing it for everyone he knew, hoping they could tell him who the "real" author was.