Quick answer: Face-on: with irons the ball sits much closer to the lead foot than the center-forward reference; the shoulders are open at address.
What it does to the ball
Contact on the upswing of the arc with irons: thin strikes and pulls; the open shoulders create an out-to-in path and a weak fade.
Root causes (by frequency)
Copying the driver ball position with every club
Aiming the shoulders at the ball instead of parallel to the target line
How to fix it
Alignment-stick station: mid irons one fist ahead of stance center, driver off the lead heel; check it on video
Drills that fix it
Ball-position station · 10 min · 15 reps
Lock in the correct ball position for each club Lay one alignment stick perpendicular to your feet pointing at the ball, and another between your feet marking the center of your stance. With a 7-iron, the ball goes one fist ahead of center. Hit 5 balls, review on face-on video, and reset.
Frequently asked questions
What is "Ball position too far forward" in the golf swing?
Face-on: with irons the ball sits much closer to the lead foot than the center-forward reference; the shoulders are open at address.
What does ball position too far forward do to the ball?
Contact on the upswing of the arc with irons: thin strikes and pulls; the open shoulders create an out-to-in path and a weak fade.
How do you fix ball position too far forward?
Alignment-stick station: mid irons one fist ahead of stance center, driver off the lead heel; check it on video
Sources
David Leadbetter — The Golf Swing
PGA of America — PGA Teaching Manual: The Art and Science of Golf Instruction