Diet. One flawed study purported that Chocolate, french fries, potato chips and sugar, among others, affect acne. A recent review of scientific literature cannot affirm either way. The consensus among health professionals is that acne sufferers should experiment with their diets, and refrain from consuming such fare if they find such food affects the severity of their acne.
But how is someone supposed to know what foods increase or decrease his acne? Try to pay attention to what he eats and what changes in diet are linked to what effects? How will he know which food did it, and how will he know what the time delay between diet changes and acne changes is? (If acne changes, was the it due to the food 2, 4, 6, or 20 days ago? Or not due to food at all?) Scientists trying to do controlled studies haven't figured anything out yet. A person who goes on his own anecdotal evidence will almost certainly be creating unscientific superstitions for himself to follow. This should not be encouraged. People have enough hang-ups about food already. The only responsible advice for scientists to give is, "don't worry about it, eat what you want."
That some scientists would encourage people to act on anecdotal evidence in this way suggests they are not competent to perform studies themselves.