Here's a weird one I've done twice:
People who are bad at being vegan/vegetarian.
I have some very faithful vegetarians/vegans (Holy Star in my novel "Damsel in the Red Dress" is one of them.)
But then I have some people who want to be vegetarian or vegan but aren't...exactly.
Texas is usually pretty good at be vegetarian, but she has a weakness for bacon. On the other hand, Jinho genuinely dislikes eating animal products altogether, but his bff/roommate/business partner does manual labor for a living and needs much more protein than the vegan diet can provide for him, so they compromised to eat poultry, seafood and dairy, but no mammals. They do still eat vegan meals pretty regularly, but they mix both of their eating habits.