My no.1
Gambling systems. I am sure there are some good systems around but at the end of then day it's gambling. I.e there is a risk. See any poor bookies or casinos?  If you do go anywhere near these avoid the cardinal sin of falling for the Martingale system. I.e. Anything that suggests you keep doubling your bet until you win, and even telling you the statistics to support it as a good idea - DO NOT TOUCH IT. You will loose your money!

Creating a business, on or off line has the same fundamental requirements.
            1  You need a good product or service.
            2  There must be a need or requirement for the product or service.
            3  Potential customers (buyers) must be aware of the product and where to buy it.

Marketing has always been about thinking up new ways to ensure that all the above three requirements are in place. Unfortunately some people are not afraid to take the concept of marketing into the immoral or even illegal.

This page exposes  and explains some of the worst examples so you can avoid them.