On a value stream I worked on once, the flow would always stop at one polishing/grinding operation. Kaizen event after Kaizen event was done in that area to make it go faster. The teams spent days upon days trying to take seconds out of the process time while ignoring the real problem. Looking at the standard work, the area was well below takt, but the flow was still jamming up - so what was going on? Well, the parts would go to the next cell and then come right back. The real problem was quality variation. Some days the rework could be 5% and other days it might be 50%. How can you really flow value around such an unstable process? You can't.
We came up with a fairly simple solution to the problem but many thought it was crazy. Add a secondary polish step to reduce the variation. No one denied that the secondary polish step improved the quality but people thought it was crazy that we wanted to increase process time by 10%. How could a Lean guy add process time!? The process was still below planned cycle time/takt time and now we created process stability. We regulated the flow and created chaos by adding a bit of extra work - well worth the price.