Lumbar degenerative spondylolisthesis (LDS) is a prevalent condition among the elderly population. Magnetic resonance imaging (MRI) is often the first investigative modality if indicated clinically. However, the standard supine position used during an MRI may fail to detect dynamic instability. In such cases, the presence of facet joint fluid is a reliable sign, and further investigation, such as stress radiographs, should be conducted to confirm dynamic instability. Here, we present a typical case demonstrating the importance of this finding. A patient presented with neurological claudication, and an MRI was initially unremarkable except for the presence of lumbar facet joint fluid. This finding prompted us to conduct stress radiographs, which eventually confirmed dynamic instability.