What's your favorite 'backwards cameo'?

I don't know if I used the right term but what I mean is this: when rewatching an older movie and being pleasantly surprised to see a now famous or favorite actor in a smaller role that you forgot or never knew it was him/her.

I watched Tombstone as a kid many many times. For some reason I kind of forgot about it.

I rewatched it now for the first time in I think 20 years and was amazed and happy to see Billy Bob Thornton as the weak and laughable bully.