[JamisonM]

Actually the mistake that set the project back the most was the opposite, from the article:

Stallman later admitted, “I take full responsibility for the technical decision to develop the GNU kernel based on Mach, a decision which seems to have been responsible for the slowness of the development. I thought using Mach would speed the work by saving us a large part of the job, but I was wrong.”

He made what he thought was a pragmatic choice that ended up being wrong, not the "perfect" kernel.