Any diet that is based on an unsubstantiated claim that "humans have not evolved to digest X" where X is something that humans have been eating for thousands of years. Paleo comes to mind, but there are others. The claim made by these diets isn't structurally invalid, but often the claims are made without any investigation and are substantiated by anecdotal evidence that people feel better when eliminating X from their diet. That's fine, but that's not the same thing as being incompatible from an evolutionary standpoint.