You're on your way out of a store getting ready to toss your groceries in the trunk when the guy parked next to you points out a dent in your car. He tells you this is your lucky day: he's an autobody mechanic who just got home from a day at the shop.
What could cost you hundreds at the auto body shop, he will do for a mere $20. The dent is pretty ugly, so how can you go wrong? You say yes.
The "mechanic" starts working on it with some tools he has sitting in his trunk. Then when you're not paying attention, he applies a white compound over the dent, telling you it will protect the new finish from rain or other surprise weather for the next 24 hours. He collects your money and heads on his merry way, assuring you your car will look good as new tomorrow.
When you do finally take off the compound the next day, you'll find the dent is still there. Many scammers will spend the whole day in the parking lot waiting for victims, before moving to a new lot.
How to avoid: never believe anybody who tries to do business in a parking lot. It may seem horribly convenient--and that's why the scam works. But don't fall for it.