What is the most common mistake while making a veneer?

The most common mistake in veneer preparation is OVER-REDUCTION (not under-reduction). Specifically, OVER-REDUCTION of the GINGIVAL THIRD is common because dentists tend to over-prepare at the margin trying to hide it. Over-reduction into dentin at the cervical area reduces bond strength (enamel bonds are stronger) and can cause sensitivity. Under-reduction of the incisal third is also noted as the preparation may not allow sufficient ceramic thickness. Note: the PDF notes 'over-reduccion' as the answer to under-reduction questions. B is correct.