I wrote a program, by hand to cut a large aluminum vacuum mold and the operator was testing it in air over the part. Boss come over and and is chewing me out for wsting time running it in air. So we lower the Z and start cutting. Right in the middle of the pocket the Z goes straight down because there is a negative Z value. To this day I don't know where the minus sign came from. Boss heard the noise and came running over. "What the hell was that!" I calmly said "I made a mistake, we are going to need another piece of aluminum" His jaw falls open and no words came out! He just turned and walked away. Everyone crouds around and one guy says "Wow, no one ever admitted a mistake like that before! Usually he stands there screaming at you for 15 minutes."