Revealing mistake: When Jamie sits down after rappelling down the canyon wall you can see that he has climbing rope attached to his harness. It's wrapped in duct tape to look like its the rope he and Adam made of duct tape. This may just be a safety line to support his weight if he duct tape rope snaps, but the Mythbusters usually point out any backup safety systems and explain that they are there for caution and don't contribute to the test. (This was explained in the aftershow. It was a safety line required by the shows insurers).