If you've ever been invited to an event or group on Facebook by a friend who has lost their phone, and is asking everyone to post their phone numbers so they can recreate their address book, you'll be pleased to see that Facebook is now prompting these people to do something less stupid.

Don't know what's actually triggering this -- presumably it doesn't show up every time someone tries to make an event. Or maybe the problem really is that prevalent, and so it does.