Batching some extra just-in-case there is a problem
Buying an extra machine just-in-case it breaks down
Having a big safety stock just-in-case there is a quality problem
The behaviors in the list above are understandable, and even appropriate, if the underlying conditions don’t change. Just-in-time manufacturing addresses the underlying reasons, so that the behaviors are no longer appropriate. When quality rises, there is no need for as much safety stock. If the root causes of problems are resolved, then the extra stock is unnecessary.
The key to Lean production is to continuously identify any of the reasons that prevent a single piece of work from flowing seamlessly through a production process and solving the underlying problems.