A production system is called improvable if limited resources involved in its operation can be redistributed in such a manner that a performance index is improved. The production systems considered in this work are the serial machining and assembly lines with unreliable machines and finite buffers. The limited resources involved are the total work-in-process (WIP) and the workforce (WF). The performance index addressed is the throughput. It is shown that the system is unimprovable with respect to the WF if and the only if each buffer is on the average half full. The system is unimprovable with respect to WIP and WF simultaneously if and only if the above holds and, in addition, each machine has equal probability of blockage and starvation. Thus, in a well designed system each buffer should be mostly half full and each machine should be starved and blocked with equal frequency. The improvability indicators, mentioned above, have been applied to a machining line at an automotive component plant. Both WF and WIP improvability measures have been considered. It was shown that the throughput of the line can be increased by a factor of 2.