When I was watching the Transformers: ROTF Sound Profile for the Reedman Scene (about 5 minutes in), there was the sound of the little steel balls rolling around on the floor and it sounded, to me, like the sound of a drinking glass or a glass bowl rolling around -- then I had to stop and figure out whether I was remembering the actual sound or just the sound that I've heard associated with that image in various films.
I was wondering, just out of curiosity, whether any of you have a hard time remembering whether a sound was real or designed, specifically when you're writing up your list of things you'd like to record that day or when you're trying to separate source elements in a film you're watching in a "how did they do that?!" moment?