Ponzi schemes
The kind of Ponzi schemer most people run into is an individual or a small firm who promises extraordinary returns from real estate or obscure, highly complex trading strategies.
It usually starts with friends or friends' friends, and makes your put down your inner alert system as you normally will ask "isn't it too good to be true"? Of course the Ponzi schemers may leave a few obvious shortcomings of their promised investments, the purpose is for you to catch them and be proud of how smart you are, and forget the real danger.
Staying out Ponzi schemes isn't that hard to do, just remember this simple rule: if it sounds too good to be true, frankly, it is.