Why this is a mistake: Think of point of view like the camera position in a film. If POV is handled badly, readers won’t know from which perspective they are viewing the scene, and they could become disoriented. A disoriented reader is an unhappy reader.
The solution: Make sure readers know what the POV is. Let’s go back to the camera analogy. Pretend you’re the film director. Where are you putting the camera to film the scene? When considering how to tell your story, the first thing you have to do is select a point of view (or multiple points of view). This may be the most critical decision after you have your original idea.
Once you’ve made your choice, your next challenge is to keep the readers oriented as to which camera they are seeing the scene through. This is especially important if you’re using multiple viewpoints. In a film, a cut is a change of camera position; in a story, the end of a scene marks any change in POV. Readers have got to know from whose point of view they are viewing the scene. Lose that clarity and you lose your readers.