@Chambabuilt - this is a sticky area, and how you wish to handle it is entirely up to you. Rather than try to out-smart dishonest customers, I simply offer no guarantees, and this is standard in high-performance and racing. The customer has no functional guarantees when s/he buys racing parts, custom pistons, $10000 connecting rods, experimental ECMs or anything else, because it is to be used in a high-risk or non-standard application or use. With best intentions and efforts, stuff happens, and professionals know it. If this is a problem for the customer, then they should not be involved in a high-risk activity, and you do not want them for a customer. How do you know? Simple. Will they sign the contract that simply states that?
If you get into the weeds with who is to blame, or try to control outcomes, it never ends and everyone loses. Was the failure due to a failed capacitor? Or an injector going lean for one of a dozen reasons? What was the octane in the tank? Who made sure the plug wires would not crossfire? The tuner provides a tune that works that day, on that engine, set-up that specific way, with the fuel in the tank that moment, etc. What happens after that is out of the tuner's control. This is where the customer must trust everyone in the chain just like they trust their machinist, or the guy that blended the fuel at the fuel depot, or the guy that crimped the connectors, or themselves if they use the wrong stuff or operate the engine improperly. Oh, wait. Performance and racing is always operating "improperly".
David
PS: Anyone can make mistakes, and you can't prevent or protect them in every way. But, remember that if they did "something stupid", THEY did something stupid.