Work in progress is treated as the mirror of waste in any system. Lean manufacturing always talks about having a work in progress free manufacturing facility. This is because a WIP free manufacturing facility has no imperfections in the process of manufacturing.

Every imperfection in the system creates a requirement for work in progress. For an example if there is a machine breakdown in the production facility, if the processes before this machine will continue to manufacture there is an overproduction. This will accumulate semi finished goods near the broken machine. This is the WIP buildup due to the imperfection in the system.

If the manufacturing process is not properly synchronized, then there is a problem of over production in some processes. Some of the processes will continue to produce more and some processes cant keep up to that phase. In lean manufacturing terms this is known as the imbalanced system. In this case obviously there is a WIP accumulation near the process which can not keep to the TAKT time of the system.

Not only Work in progress is a mirror of waste, WIP itself a waste. For an example it hides the quality defects in the manufacturing facility. Lets say we have a work in progress of a day. This means a part manufactured today will go in to the next step of manufacturing only by tomorrow. If there is a problem in the process, it will be revealed only by tomorrow. Meanwhile there will be a day worth of production as semi finished goods in the process. All of these semi finished products might have the same problem. In that case a full day of production is wasted and worst will take days to be corrected.