If we're going to say that "Stephanie" and "Eyes of the World" are so similar, we definitely must acknowledge the similarities between "Eyes of the World" and "Bang the Drum"! Then obviously there is the early demo where Stevie sings parts of "Enchanted", so it's possible SHE used some of the song for another song and I also read somewhere that in that original version, the drum beat was going to be used for "Bwana", but Lindsey decided to use it for "Eyes" instead. It's a popular tune!
As far as others go, I think there are at least four songs on SYW alone that have riffs from other recognizable tunes. Here is a thread from long ago regarding that:
http://ledge.fleetwoodmac.net/showthread.php?t=9596