The known knowns, known unknowns, and perhaps even the unknown unknowns of why a calorie is not a calorie.
Where are your limits? Inspired in part by mikhail's earlier post on the gelatin used in Guinness (and Bass), for those with voluntary diet restrictions (kosher, halal, vegetarian, etc.), what unexpected choices have you faced? Does it go beyond food? Toothpaste? Collagen injections? Silk? Buying a car with leather seats? A used car with leather seats?