I'm interested in this part: "Steele and Sussman tried to start over when they first began working on Scheme, but they seem to have been practically the only on

I'm interested in this part: "Steele and Sussman tried to start over when they first began working on Scheme, but they seem to have been practically the only ones. And they made, at least from the point of view of brevity/power, some serious mistakes early on." Paul, out of curiosity, what are those serious mistakes?