Letting the fear of missing creep into their mechanics. Everyone does this to some degree, especially on pressure-filled, difficult shots. But for many players, it affects almost every shot. And it causes an entire cascade of bad results--grip tightens, tip goes askew just before contact, head lifts up, elbow drops, body english, etc. I think it's most prevalent among lower-intermediate players who are trying to improve and are aware of their weaknesses but haven't quite developed the confidence/mechanics to start pocketing balls consistently.