He put a crown on the wrong molar, grinding down the molar that was above it in the process. This…read morelead to an unbalanced bite and a very sensible molar because he ground half of it down, which caused jaw issues.
A year before this happened he seemed to be very much aware which molar needed the crown, though presumably due to a lack of record keeping he decided the crown belonged on an entirely different molar.
All this led to various complications & pain which I deal with every day.