"As the World Falls Down" from Labyrinth.
Interestingly, for a long time, I found the ballroom sequence to be one of the most boring/least interesting parts of the movie. And for a long time, I couldn't remember if that scene had a song or just instrumental music. I would watch the scene and then forget shortly afterwards if it had lyrics. I have remembered for about half a year now.
And then last week, I decided to watch the clip on YouTube... and I;ve found myself fascinated by it. As if I was in a trance. And I suddenly decided to keep rewatching the clip over and over again. And then it's been in my head for the last four days, whether I was watching the clip or not.
At least I don't think I have to worry about forgetting if that scene had singing or not anytime soon.