Biggest Rookie Mistake and Biggest Long Term Mistake
My biggest Rookie mistake was recoil anticipation. For YEARS all my handguns fired low and left (I'm right handed.) Finally a good coach had me work on Timed and Rapid Fire with the goal of keeping my eyes open when I fired. It worked, finally. When I could see the outline of the bullets going downrange - in good light - I knew I had that one licked.
Biggest Long Term Mistake was trying to 'squeeze' the shot too slowly. Another good coach told me to basically just keep 'mashing' on the trigger no matter what in Timed and Rapid Fire. I didn't believe it at first, but trusted the coach and sure enough, my Timed and Rapid Fire scores went up. Then I had to learn to apply the same technique in Slow Fire. My Slow Fire scores went up as well. A proper trigger pull is much like ringing a door bell; not a wild slam, but a deliberate and steady pull (squeeze, mash, press) on the trigger.
Of note, learning to fire a double action revolver in double action by a steady long pull aided in learning the technique for single action with either revolver or pistol.
As with all skills, they require practice to learn and practice to keep them up.