The cingulum height needs to be lowered to allow space for the metal, if not, the lab will try to bulk it up to prevent it from being so thin.

ERROR

1.) The operator failed to lower the cingulum height. So naturally, he did not compare it with the contralateral tooth.

PREVENTION

Always check with the contralateral tooth. When you see some shiny surfaces, it means this surface has not been reduced yet. So always check if there are any. It is a sign that you still have an inadequate reduction.