Until I watched this from German teaching pro Florian Meier...
I believed that all errors were the same because they produce the same result--a lost point. But what Meier makes clear is that among the three ways to miss--short, wide, and long--one in particular exposes you to the least harm.
Awful: missing short. Obviously, a ball in the net is an immediate lost point. But even short balls that clear the net create a problem: an easy shot for your opponent and less time for you to react because of his close distance to you.
Bad: missing wide. First, there's a much smaller margin for error when aiming for the sidelines...as the court is about half as wide as it is long. Next, unless you're aiming for the absolute corner, you're usually placing the ball relatively short in the court...again spoon feeding your opponent with an easy ball. But finally, a wide ball gives your opponent the luxury of angles...angles which expose open court on both sides of you.
Acceptable: missing long. Florian notes that the pros miss long far more than they miss short and wide. And it makes sense after we looked at the consequences of aiming short or wide: you put yourself in a vulnerable position in both cases. But deep shots placed near the center of the court limit your opponents options, give you more time to react to his return, and are the most likely to produce a weak shot.
**From my recent blog post on singles strategy: