One type of faulty registration is called Mixed Registration.
If we use an analogy of the house and us going to work, this condition can be described as occurring when the muscles seem to forget what their jobs are and where they live. We find that the bottom of our voice is too small and light, maybe even breathy or a feeling of constriction with us unable to sing even with a normal volume. Then, it seems that the upper part of our voice is too big and heavy with us having to really push and sing loud to get it to happen.
In healthy conditions, when we are singing at the bottom of our voice, the cords are bigger – more length, width, depth. At the top of our voice, the cords are smaller – less length, width, depth.
What has happened in a mixed registration is that this natural state of affairs is reversed with the cords smaller on bottom and bigger on top, like a pyramid turned upside down! If a mixed registration is not corrected, then what typically happens is the singer begins to lose notes at the top and at the bottom with the range shrinking to a few notes in the middle; seemingly stuck in a little box.
The cure is for the registers or muscles to be separated, isolated, and worked independently of one another so that they relearn where they live and what is the job they are supposed to be doing. Then, the pyramid will rest once again on a broad foundation with the apex at the top.