My buddy did that with a transmission swap, pocketing the difference. The car ran fine for a year before the guilt ate him up and he told the owner. Sometimes the repair holds, but your conscience doesn't.
Wait, he actually kept the extra cash from the transmission job? That's downright shady, even if the car ran okay. Stuff like that eats at you because you know it's wrong, no matter how long it takes. A whole year of pretending nothing happened must have been torture. Our minds really don't let go of guilt, do they? It's like a slow burn that just gets worse. Has a friend ever told you about a secret they were keeping?