Learn the basics first, then discover the subtleties...
I briefly mentioned yesterday that some principles get embedded into players' minds as they learn the game. Many times principles represent the safest action in a given situation. That does not mean that the principle represents the only option. My favorite contrarian, Mark Rennesson, discusses some mistakes that mid-level players make, including the exposure of some of the principles as myths, in his article 5 Intermediate Mistakes. An excerpt is shown below.
Pickleball is considered an 'open-skilled' sport because it requires wide-ranging and varied skills and the circumstances are always changing. The height, speed, spin, distance and direction a ball is hit is never quite the same as the ones before it. The fact that players are standing at (or moving through) different places on the court, that wind or sun conditions change throughout a game, means that pickleball is an activity full of variability and change.
That said, as someone who works with a lot of pickleball players, I can say that for all the variability in the sport, there are quite a few mistakes and habits that are common to intermediate players. Here are a few of them...