Scam: This one is the worst one because it is done by legitimate businesses. Consumers are offered "free gifts" either through pop-up ads, junk mail, or spam phone calls. By accepting these gifts, consumers are enrolled into some kind of "discount club" or "travel club" which will end up charging them mebership fees if they don't cancel it. Usually this whole process isn't made clear to the consumers. Personally I've had to cancel a whole bunch of these deals for my parents.

How to protect yourself: Remember there is no such thing as a free lunch. Tell your banks and credit cards to stop offering you these gifts.