Buffers decouple fluctuations in the material flow. It is common wisdom in industry that a full buffer indicates a downstream bottleneck and an empty buffer indicates an upstream bottleneck. Numerous different bottleneck detection methods use this approach to detect the bottlenecks. However, so far this common wisdom on the shop floor has not yet been verified academically. The authors tested this hypothesis using a bottleneck detection method that was able to detect the bottleneck in a system at any given time. The bottleneck direction can reasonably be determined based on the inventory levels of the buffer only for symmetrical systems. In asymmetrical systems, the likelihood of the bottleneck direction is biased toward the bottleneck.