The thing some email clients do where they say “did you mean to attach something” is the one I love.
I’d like a Mark II version which also says “there is no such thing as Sunday 8th of August this year” so that I can fix my errors before they cause problems.
It's just fundamentally a bad idea. Software should never, ever bother the user during use about how it's being used. That's what tutorials and documentation are for.
No matter how clever and subtle you think you're being, you're still just redirecting the user's attention from the task at hand toward the UI itself. No matter how ignorable, dismissable, or optional it is, it's an abysmal UI failure and it shouldn't even happen once. Make the feature right in the first place and you won't have to resort to nagging.
