You're on your way home from work for a long weekend and you feel like a million bucks. So much so that you almost don't notice the red light telling you you're running on "empty."

You pull into a gas station and a well-dressed man standing by another car strikes up a conversation. He tells you about this amazing new liquid formula that saves you a big pile of money on gas.

You make a note of it and are just topping up your gas tank when another man walks by. This guy's dressed in coveralls on his way to the gas station. He overhears the conversation and agrees with the well-dressed man. He's a mechanic, he tells you, and he swears by the stuff. Just a few drops and it can extend the fuel for another third of a tank.

As a matter of fact, the mechanic happens to have a couple bottles in his truck, if you're interested. You look up at the pre-long weekend price gouge and you think, "why not?" You pay the guy and think is your lucky day. It isn't. The liquid could be a mixture of things, but it certainly isn't the magic substance that you are hoping for. 10-20 bottles sold at a few different gas stations and the scammers can walk away with $1,000/ day.

How to avoid: never buy anything from someone in a parking lot, especially when you've never heard of the product. Just remember the product name and read a review online or ask the person where you can buy it.