Less severe, but more pervasive and longer lasting than some of the safety-violation examples given, is people simply being afraid of the gun. This is particularly true with handguns.
People struggle with racking the slide because they are afraid they will damage the gun if they grip it firmly, or that it will cut them if they grab it hard. They will take to heart the OP's point about not getting their hand directly behind the slide (solid advice), but become so fearful of the slide's motion that they simply cannot bring themselves to get an appropriately high grip with their firing hand, and won't come anywhere close to high enough with their weak hand. Similarly, people fear the recoil and many of them will simply not apply any pressure with the weak hand (even though this is where most recoil control comes from with modern/current pistol technique, they fear that a firm connection with the gun will wrench their wrist or elbow or something), which of course means the gun jumps around even more.
Handling the gun correctly requires the right balance of caution (muzzle and trigger discipline, primarily) and confidence. Similarly, firing the gun requires the right combination of firmness (positive grip) and neutrality (not pushing the gun around before it goes off). In many ways, the art of pistol shooting is about progressively narrowing the gap between those two extremes, doing as much of each without compromising the other as possible.
. Needless to say that was her last shot. 
