Overstabilization
Excess spin that keeps a bullet's nose from following the trajectory arc downward, which can shave a small amount of effective BC.
Overstabilization happens when a barrel spins a bullet much faster than it needs to, driving the stability factor well above the comfortable working range. The penalty is that an overly stable bullet resists pitching its nose down to stay tangent to the falling arc of the trajectory, so it flies at a slight nose-high angle through the descending portion of flight. That small angle of attack adds drag and can trim the effective ballistic coefficient, costing a little drop and drift performance at long range.
In practice the effect is modest and far less destructive than the opposite problem of instability, so most shooters accept a healthy margin rather than chase the minimum. A fast twist rate paired with light, short bullets is the typical recipe, since the high spin rate outpaces what the projectile requires. The goal is a comfortable buffer above marginal stability without piling on spin you cannot use.