It should go without saying that Tim Horton doesn't add nicotine to their coffee to get you addicted or that the government isn't adding fluoride to your water to control your mind. Of course, that doesn't mean people don't believe these things.
Strangely though, some consiparcy theories are surprisingly accurate -for example, the popularity of fondue in the seventies was all a result of brilliant marketing from the Swiss Cheese Union. It sounds absurd, but it's actually well documented.
You can read up on more of these odd theories over on Thrillist.