One of the most common fingering faults in younger players is Eb in the middle register…many students keep the first finger of the left hand down on this note…worse yet, their ears are so used to hearing the note like this that they have a hard time hearing the mistake. The first thing a teacher needs to do is help them hear the difference in timbre and pitch so that the student can catch themselves in the mistake, and eventually catch themselves fingering the note correctly every time!
I believe faulty hand position is a major culprit to blame in this fingering fault. Balancing the flute against the knuckle joint of the left hand first finger will prevent the student from having to squeeze the flute with the first finger of the left hand just to keep the flute from rolling away from them. Fixing faulty hand position, and enforcing the correct fingering every time are both important steps to enforce in lessons and at home practice.
Likewise, the pinky on the right hand is often used to hold the flute …and lifting it for D in the middle register becomes difficult. Fix this problem by allowing the right hand thumb to balance the majority of the weight of the instrument so that the right hand pinky is free to lift for D, and slide to the foot joint for all the lowest register pitches. This will also alleviate any chronic pressure on this finger, which is a common issue for students used to using the right hand pinky as a fulcrum to hold the flute. Patience and consistency will yield the consistent and correct results.