Wandering grips and open rackets
One of the more common problems I see with beginners and some intermediate players are what I call "wandering grips." I wrote about grip problems in this week's Tip of the Week, and this is a related problem. It seems that no matter how many times I remind them, some players change to a bad grip as soon as the rally begins. They aren't even aware of it. The reason is that they have unfortunately learned to hit the ball with the bad grip, and this has led to bad stroking technique. Since this bad technique has become habit, they automatically use this bad technique as soon as the rally begins - and so they subconsciously switch to the bad grip that matches the bad technique, since they cannot do the bad technique with a good grip.
The solution, of course, is lots of repetitive practice with the good grip and good technique in a controlled practice situation, usually multiball training. When the good technique seems ingrained in such rote practice drills, then the player can try them in more random drills, and finally in practice matches.
In theory, that's all you have to do to fix these problems. In practice, there are times where I'm busy smiling and being patient on the outside, but on the inside I'm screaming, "For the zillionth time, fix your grip!"