An accurate minimum miscibility pressure (MMP) is one of the key factors in miscible gas flood design. There are a variety of experimental and analytical methods to determine the MMP, but the most reliable methods are slim-tube experiments, 1-D slim-tube simulations, mixing-cell models, and the key tie-line approach using method of characteristics (MOC). Direct comparisons of all these methods generally agree well, but there are cases where they do not. No explanation has yet been given for the anomalies, although the MMP is critically important to recovery. The focus of this paper is to explain when current MOC results may not be reliable and how to identify when this is the case. We demonstrate using fluid characterizations from Middle Eastern oils that the MMPs using the MOC method can be over 6500 psia greater than those calculated using a recently developed mixing-cell method. The observed differences in the MMP increase substantially as the API gravity of the oil decreases. We show that the key tie lines determined using MOC methods do not control miscibility for such cases. We explain the reasons for these differences using simplified pseudoternary models and show how to determine when an error exists. We also offer several ways to correct the MMP predictions using the MOC for these complex gas/oil displacements.