Falling prices may be a 'warning sign' of a recession. [=may indicate that a recession is coming]
There were storm 'warnings' [=warnings that a storm was approaching] issued for the area.
I'm giving you 'fair warning' that you had better not discuss politics at dinner tonight.
(..) - Why don't you want to talk about it? - Why do you want to? - Why are you blocking? - I'm not blocking. - Oh, yes, you are. - Donkey, I'm warning you. - Who you trying to keep out? - Everyone! Okay? Oh, now we're gettin' somewhere. Oh! For the love of Pete! What's your problem? (..)2001 Shrek