Overproduction is one of the seven wastes in Lean. It is the act of making a product or performing a service before the downstreamcustomer asks for it. Overproduction is one of the leading causes of excess inventory.
Overproduction is prevalent in push systems where upstream processes build according to a schedule, and then send the product out regardless of whether their customer is ready for it. Lean deals with overproduction by creating pull systems, where products are only built when the downstream customer asks for them.
One of the barriers to pull, and hence a driver of overproduction, is long setup time, which increases batch size. Lean deals with this form of overproduction waste through SMED (or single minute exchange of dies).
Taiichi Ohno, the father of modern Lean, believed that overproduction was the worst form of waste, creating or hiding all the other forms.