Revealing mistake: In the scene where the wife comes in to the morgue to view her dead husband, who she helped to euthanize, the dead man is breathing throughout the whole scene. You can see the sheet rising and falling at his stomach. (00:22:15 - 00:24:05)
Revealing mistake: When MacPherson knocks Joshua off the platform, he falls on a bunch of pipes, but they are already bent before Joshua actually lands on them.
Revealing mistake: The bullets the priest takes out of the ammunition tray have no primers. (01:01:33)
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).