Quick answer: A miss pattern almost always on the low side of the hole; the player sets up and hits without looking at the putt from behind the ball or from the low side.
What it does to the ball
Putts that die on the low side of the hole: the read undercuts the break and the ball never had a chance to go in.
Root causes (by frequency)
Reading only from behind the ball, never seeing the slope from the low side
Aiming at the hole on breaking putts instead of at a spot to one side
Choosing the line without first deciding the speed (line and speed go together)
How to fix it
A two-stop reading routine: look at the putt from behind the ball and from the low side, pick a concrete breaking spot and putt to THAT spot, not to the hole
Drills that fix it
Read from behind and from below · 15 min · 12 reps
Turn green reading into a two-stop routine On a breaking putt, read it first crouched behind the ball, then walk to the low side of the hole: from below, the slope looks twice as clear. Pick a concrete breaking spot (a mark, a shade of grass) and putt to that spot, forgetting the hole. Note how many balls die on the high side: that's the sign your reads are improving.
Frequently asked questions
What is "Not reading the break (slope and grain)" in the golf swing?
A miss pattern almost always on the low side of the hole; the player sets up and hits without looking at the putt from behind the ball or from the low side.
What does not reading the break (slope and grain) do to the ball?
Putts that die on the low side of the hole: the read undercuts the break and the ball never had a chance to go in.
How do you fix not reading the break (slope and grain)?
A two-stop reading routine: look at the putt from behind the ball and from the low side, pick a concrete breaking spot and putt to THAT spot, not to the hole
Sources
Dave Pelz — Dave Pelz's Short Game Bible
Dave Stockton (con Matthew Rudy) — Unconscious Putting