Ohno believed that the waste of overproduction was the most serious of all the wastes because it was the root of so many problems. Overproduction is making too much, too early or "just-in-case". The aim should be to make exactly what is required, no more and no less, just in time and with perfect quality. Overproduction discourages a smooth flow of goods or services. "Lumpiness" (i.e. making products or working in erratic bursts) is a force against quality and productivity. By contrast, regularity encourages a "no surprises" atmosphere which may not be very exciting but is much better management.

Overproduction leads directly to excessive lead time and storage times. As a result defects may not be detected early, products may deteriorate, and artificial pressures on work rate may be generated. All these increase the chances of defects. Taking it further, overproduction leads to excessive work-in-process inventories that lead to the physical separation of operations and the discouragement of communication.

Yet overproduction is often the natural state. People do not have to be encouraged to overproduce; they often do so "just to be safe". Often this is reinforced by a bonus system that encourages output that is not needed. By contrast, the Kanban system prevents unplanned overproduction by allowing work to move forwards only when the next work area is ready to receive it. Although kanban was made famous in manufacturing, it was originally developed from the supermarket restocking procedure and certainly has application in the service industry. (Hamburgers are only made at a rate in line with demand and clerical operations are most effective when there is a uniform flow of work.) The motto "sell daily? make daily!" is as relevant in an office as it is in a factory.

Overproduction should be related to a particular timeframe. At Toyota, overproduction is considered to have occurred if the daily schedule is exceeded. Most other companies should consider a week as the no-overproduction target.

Quality guru Joseph Juran noted that in Japan in the 1950s there were severe disruptions in power supplies, with production sometimes possible for only 3 hours per day. In such circumstances making the wrong product, or making it too early, or working on an already defective item was something to be strongly avoided. Likewise the transport infrastructure was awful. Transporting the wrong item, too early, or if defective was also a huge waste. It was in these circumstances that Taiichi Ohno developed the JIT system, and led him to conclude that overproduction is the worst sin of all.

Examples: "lumpy" flow, production above target, excessive lead-time, and delivery too early.