Fran Drescher's one scene in Saturday Night Fever comes to mind. It's also bizarre not only seeing her randomly pop up in the movie but her voice also sounds surprisingly normal compared to how we know she'd sound like.
Also Maya Rudolph was in both As Good As It Gets and Gattaca in the same year. And Jenifer Lewis was in Beaches as basically a featured extra.
What about you? Can you think of any examples of celebrities popping up in iconic films before they were famous? (Therefore please don't list examples of movies that made people famous or helped kickstart their careers)