I took my car in because it over heated

I'd be upset too that your mechanic wasn't psychic and that they didn't try to sell you something you didn't need. It's amazing that no matter what your mechanic does or doesn't do, you blame him. He did some preliminary diagnosis, came up with a solution, and the car ran fine, ... So you expected him to keep looking! When we DO keep looking, you blame us for trying to sell you stuff you think you don't need. You can't have it both ways. Had he a reason to suspect a leaking cylinder head gasket, there are additional tests he could have performed to verify that, and it would have cost you more money. Obviously it wasn't leaking very badly yet, but where do you suppose your coolant went originally? The head gasket had already started to leak, but very slowly at first, which is typical. A leaking head gasket often results in engine overheating, AND, an overheated engine can result in a leaking head gasket or it can make a leak worse, as happened to your engine. BY your own admission there was no problem for 20 miles, just like when your mechanic had it. Why do you think he should have kept on looking when there was no problem? Your mechanic didn't cause the problem, and he likely had no reason to suspect it was leaking, so give him a break.