The article aims at indicating limitations to the effectiveness of the monetary policy other than the liquidity trap that has been widely discussed recently and is identified nowadays with the ceiling of zero nominal interest rates. The identified limitations include primarily a discrepancy between price stability as a goal of the central bank and the central bank's ability to use monetary policy (interest rate policy) to counteract the decrease in output resulting from demand or supply shocks. The article also points at the cost of wasteful use of capital in the case of negative real interest rates. Due to this cost, the policy of counteracting the decrease in output by lowering interest rates may not be not optimal in terms of the social welfare. Within this framework, the model presented in the article enables to explain the phenomenon of stagflation as a result of a negative supply shock. The conclusions drawn from the model also indicate that counteracting the increase in consumer spending (the decrease in propensity to save) and its inflationary effects with the monetary policy leads to a decrease output. The observation has been developed in order to analyse the optimal economic policy mix. It leads to a conclusion - in line with the Solow's golden rule - that the optimal mix is assured by the fiscal policy coherent with the zero natural interest rate. The model well expresses the "ambiguous role" of saving, indicating that an increase in propensity to save allows to lower the interest rate that guarantees price stability and to generate an output growth. However, it poses the threat of shaping the real natural interest rate. The general conclusion which can be formed on the basis of the conducted model analysis is that a monetary policy restricted by the requirement of price stability cannot be used to counteract recession phenomena, whereas tightening the monetary policy in order to defend price stability will lead to a decrease in output when confronted with inflationary demand pressure production. Based on the model, a conclusion has also been drawn that cohesion of saving rates in all its parts is a prerequisite for an optimal currency area.