Crystal failures are always a problem. We used to chuck a bag of them in a tumbler for an hour then on a tray on a vibration table for an hour. About 20% would fail on test from being entirely open circuit to being way off frequency.
That's physically almost impossible. Even if the crystal sheet were to drop off the mounts it would have to do something electrically hard to explain to cause a short. The contact areas are usually the only conducting parts on the crystal itself. Failing open is easy for the exact same reason: if the crystal is mechanically damaged that will usually dislodge one or both of the contact areas. This is because those are the only places where something touches the crystal to give it as much freedom to vibrate as possible, any kind of further constraint would impede function.
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