I notice every time somebody gets a new phone because it says "Your safety number with x has changed" but whenever I've spoken about it with friends, they have no idea what it means. An additional sentence could help here, such as "You might want to double check that you're really talking to x" or the classic "It's possible someone is doing something nasty". Although I understand that this would definitely scare a lot of people, maybe even push them into thinking Signal is insecure.