Informal norms dictate appropriate behaviors without the need of written rules and so may be difficult to learn when you are new to or not familiar with the culture.
Another example of a folkway norm that came to mind is chewing with your mouth closed when around other people, especially at a restaurant. I know for a fact this is a cultural norm in the United States, but I'm not sure if it is elsewhere. It's considered a folkway because violating it is generally harmless, perhaps only a little awkward or considered gross to the people you're around. I learned about this norm the hard way when I was a kid. I absolutely loved to chew with my mouth wide open. My mom would yell at me and tell me to "stop smacking". She'd further it by saying that I most certainly should not do that if I'm on a date with someone. I'm chuckling as I write this, because in hindsight, it is pretty funny. But had my parents not taught me that, I would have likely learned it through a more embarrassing situation. If this norm is violated, the result is usually harmless. Someone might chuckle and tell the person their chewing is loud, or calmly say to please chew with your mouth closed.