Here'a a real-life hypothetical for your torts lecture.
It's nearly Christmas and you have no money to buy presents for your kids. So, you sell your car for cash and go to WalMart to get what you can. It's 2:00 AM after an exhausting day and you finally get your purchases to the checkout counter. When you pay for your kids' Christmas gifts with the $100 bills you just got from selling your car, the cashier tells you they are fake and tears them up. When you protest, she marks the bills with the counterfeit detecting pen and the pen fails to detect a fake. It doesn't matter. The Manager has determined the bills are fakes. They call the police. You are told to wait. In the meantime, WalMart employees engage in loud public conversations about your attempt to pay for goods with counterfeit bills. The police arrive, inspect the bills and deem them to be real. The manager argues with them. Finally, the manager tries to give you back your ripped up bills. The police insist that that the manager give you replacement bills. Finally, at 5:00 AM, you leave the store.
The subsequent lawsuit is based in Intentional Infliction of Emotional Distress and False Imprisonment. Read the complaint, here.
Example of the type of counterfeit detecting device that WalMart uses but apparently doesn't trust: