The most over coached action in baseball is the arm action; because most of the time it is "taught" wrongly. When pitchers learn how to move their center of mass correctly, hip rotation back to front, control their glove side and time it all up, and their arm action will often find a cleaner path. Also, through long toss and weighted ball drills, the arm again will find a more efficient way to put force behind the baseball. The best way to teach arm action is get the entirety of the body moving together--better, and let the pitcher throw, a lot to find their way.
Arm inefficiencies can often be fixed by simply timing or speeding up other movements in the delivery. It gives the arm less time to screw up.