I can't tell you how many times I've caught juniors trying to pass off unmodified SO answers as their own work. It's rather alarming to me because they have this attitude that popular things can't be wrong, and public things don't need attribution. But knowing, that people often approach life in this way, is half the battle.
Lesser known is that SO content is generally under a CC-BY-SA license, which means not only attribution, but also "share-alike", which generally means releasing derivative work under the same license...
There is significant legal risk if your juniors are pasting non-trivial code from SO into your codebase verbatim...
