Everybody that does this has done it. Lot of reasons but usually just being dumb. No one is immune from mistakes. Problem with these cars is the expense of a screw up. The Toyota guys just go to the parts department and grab a head complete with valves for a couple of hundred. A guy that used to work for me did it on a C4 whose heads had never been off. He was a good mechanic but it was one of those days. Before it was over that block had a couple of thousand miles on it finding someone adventurous enough to machine the studs out of the block because they were so stuck. It cost me a Ferrari to fix it. All for a job that was only going to cost about $3000. That is one reason I get just a little upset at the a-holes who question why these cars cost more than a Toyota to fix. I really have no time for anonymous people who are perfect.