The risk of omission, or what we don't do right, is usually far more dangerous than the risk of what we do wrong (poor quality). Poor quality generally announces its presence during inspections and tests (internal failure) or, in the worst case, when customers complain about it (external failure).
This leads to corrective and preventive action (CAPA) to remove the root causes of the problems in question. The other six Toyota production system wastes are, on the other hand, asymptomatic. They do little or nothing to announce their presence, and can hide in plain view from people who are not trained to recognize them. Henry Ford, Taiichi Ohno, and Shigeo Shingo all described the asymptomatic nature of these wastes.