The occurrence of an S-shaped polarization curve in a simple model for the continuous electrochemical oxidation of CO on a platinum electrode is discussed. In the model, the S-shaped polarization curve is caused by the competitive Langmuir-Hinshelwood mechanism between surface-bonded CO and OH. The reaction is studied experimentally on single-crystal platinum rotating disk electrodes in perchloric and sulfuric acid solution, and it is shown that the voltammetry is in good agreement with the model predictions. When studied under current-controlled conditions, a fast galvanodynamic scan indeed suggests the existence of the S-shaped polarization curve. At lower scan rates, however, irregularities and small-amplitude irregular fluctuations or oscillations in potential are observed. Very regular potential oscillations under current-controlled conditions are observed only on Pt(111) in sulfuric acid. The possible origin of these irregularities and oscillations is discussed in relation to the existing theories of electrochemical instabilities.