Valves sticking in guides

So the customer comes to me and say's, "I had these heads rebuilt by another shop and a valve stuck. So I took them back and they redid them and a valve stuck again. I took them to another shop had them check them out and redo them and I have another stuck valve. Will you please find out whats wrong?"

Well everything measured good but two valves were bent.

Solution for the customer: Take the keys away from your 18 year old son, 'cause he is the only one driving when they bend.

Year later the son's car quit moving in an intersection. There were pieces of bell housing and clutch on top of the engine. He still had the keys I guess.