The internet is never short of a crazy conspiracy theory, and one of the craziest is the theory of chemtrails. Some people believe that the vapor trails left in the sky by planes are actually chemicals, used to control the population. When you point out that the trails have been there for years, they deny it.
They'd have some difficulty denying this. There are the trails of fighter planes in the sky above London, after British pilots have gone head to head with German bombers during World War II.