If they screw up the third time, I'd be looking for a different dealer. By now, if they can't figure out that you will inspect the vehicle with an even finer toothed comb, then they do not deserve your business. I am NOT a car dealer, but I am a business owner. Like every other human out there, I make mistakes too, but never the second time with a given customer. In fact, if I owned the dealership that tried to sell you the truck with the dent, I would have personally inspected the vehicle with the assistance of my service and body shop mangers to ensure you would not be able to find a single thing wrong....and I would have instructed the salesman to contact me the moment you arrived to so I could tell you in person that 'I' checked it over, and if I missed anything, I want to know about it direct from you, because I want your business and am prepared to prove that statement. That way, even if I screwed up, you are likely to want to complete the transaction after I do what is right....to make it right.
YOU are the customer with cash that they want. Make sure they earn it, but be respectful. When it comes to service, there is no substitute to a great relationship with the dealer.