If the distance between the steady state nominal rate and zero is small, there is not much scope for cutting rates to defend price stability in the face of a negative shock.
If the aim is to maintain inflation within the price stability range, and avoid deflation, the central bank may find itself out of standard monetary policy ammunition in the face of falling prices. Of course, the central bank can always activate more unorthodox instruments of intervention, even when it exhausts the space for further rate cuts.