You are ready to buy a car and you schedule a visit to the dealership. You don't exactly buy a new car every week, so you decide it's a good idea to bring along a friend who has auto experience. You ask him to give you a signal or intervene if the salesperson is trying to take advantage of you during the negotiation.

You pick out your car and the three of you head into the sales office for a more detailed discussion. The salesperson presents you with all the terms and gives you a few moments to discuss. When he leaves, your friend tells you all of his reservations about the deal. After 3 minutes, the salesperson returns. When he comes back in, the discussion goes really well: he knows exactly what to tell you to get you to buy today, and you do.

What you didn't know is that he left the intercom on and listened to your entire conversation in the next room. You may have been able to talk the car down even further if he didn't already know your hand.

How to avoid: it's not a bad idea to leave the dealership and come back--or just go for a 5-minute walk with your friend.