My friend was telling me (so not sure if it's true or she's heard from someone else) that if you use certain buzz words in an email, you could have the police knocking at your door and/or searching your house. 
I knew that emails weren't private but didn't realise such a system of alerting the authorities was in place.
From what she said, I think it's mainly linked to threads of possible terrorism but not totally sure.
I am glad this isn't common knowledge as I'm sure there would be some teens who would do this deliberately just for fun! 