Over-production in layman does not seem to cause a problem, actually will increase the existing stock product. So at any time if needed can be sent directly to the customer

Over-production here could be interpreted to produce the product does not comply with the request eventually makes the excess stock. This excess stock will cause the problems that have to do with quality.

In the context of lean manufacturing, over-production is the waste which must be removed from production systems. The principle of lean manufacturing is to produce products according to customer specifications and demand. If the product exceeds the demand may be indicated occur extravagance.

Waste caused by excessive production.
1. The use of area stores will narrow as to accommodate the goods ordered items waiting. The more stock the excess then place the more narrow and will certainly make the production unstable. Additionally, it will be found the goods that are not in accordance with the place. The function of 5S disturbed can not be controlled anymore.
2. The quality of goods will decline in line getting his old stuff is stored. Even if the product is treated well, then there is the additional cost of treatment of these goods, whether the cost for labor and equipment.
3. The use of inefficient engines. How should not be able to produce products that truly urgent and asked the customer, but is used to produce products which are stored only in store.

So in conclusion, lean manufacturing is an effort to reduce and eliminate waste. Product only in production if there is demand from consumers. Products that are produced only in accordance with the specifications of the customer demand. If you must produce the goods should only goods of high repeat orders, and even if it is truly a production machine is idle and aims to improve the efficiency of the engine.