common FAULTS in FACINGS
There is a number of faults that you will meet every now and then in mouthpiece facings. 'Faults', in this context, does not necessarily mean that the mouthpiece doesn't function. It may, it may even do so quite well. Yet from the prespective of the craftsman, they're still to be considered as imperfections.
Let's start to recognize the peculiarity that a fault from the one perspective is just a characteristic from another: I've seen a number of mouthpieces which where the favorites of their owners, musicians of name and fame, in which some imperfection or other could be discerned. The wooden mouthpiece might serve as an example. It tends to warp and thereby might produce a torsion in the facing. But this doesn't necessarily render it unusable. It depends on how well the curve of the facing is conserved in the process of warping. So the first question always is: «does this mouthpiece work for me?», and if not, well then, consult the list below.
I will start with three faults that might still produce a working mouthpiece and end with three that will surely lead to difficulties. The red lines indicate the faults under discussion.
- hollow mouthpiece table. The hollow mouthpiece table is sometimes made intentionally. It is helpful in securing the 'take-off' of the reed at the point where the lay departs from the table, but the ligature also introduces a stress in the reed. The opposite imperfection, see third below, is certainly much worse.

- torsion. As said above, torsion is not uncommon in wooden mouthpieces, but also is found in ebonite or metal pieces. Provided the curve of the facing is (still) allright, it need not be a reason to turn down the mouthpiece. It seems, we even might not be aware of a gentle torsion.

- flat mouthpiece tip. Now it becomes more awkward. This is in fact a grave mistake: it will make the reed bump into the mouthpiece tip before it is due, producing an overly sharp and harsh sound. Yet, when still delicate, this imperfection might produce a facing which blows easily and with a bright sound. I've known a musician who loved such a facing.

- downward bend of the table due to faulty flattening. Here the part of the table which encloses the window is already no longer flat. This is a common fault in tables which are 'flattened' by grinding them on sandpaper, a sheet of glass added or not. It makes the facing boundlessly long. The embouchure has to close the resulting slit between reed and mouthpiece before any sound is produced, producing a vague and often difficult response in the lower range.

- mouthpiece tip ' hanging down'. Considering the fact that the drawing is upside down, you might also call it a mouthpiece tip that is 'standing up'. It is the nasty version of the flat mouthpiece tip above, found mostly in metal pieces due to the mouthpiece's resistance to the grinding of the facing. Sometimes also caused by accidents of falling (in metal mouthpieces only). On both sides it leaves a narrow slit open between reed and mouthpiece, while the reed tip again bumps into the mouthpiece before it is due.

- mouthpiece tip ' standing up'. The opposite fault also occurs. It necessarily leads to a mouthpiece that won't play pianissimo (if it plays at all), blows heavily and constantly produces wind-to-the-tone.