The differential maximum bubble pressure method for determining surface tensions is often used with simplified theoretical equations which allow for simple and rapid measurements. While this approach frequently yields surface tensions accurate and precise enough for practical applications, two cases where accuracy suffers significantly are discussed. One source of error occurs when a term involving density has been neglected even though significant density differences occur between calibration standards and liquid samples. Another source of error enters when equilibrium surface tensions are determined from dynamic measurements in which only one capillary bubbling rate is varied. This occurs when the larger of the capillaries is considered to principally correct for immersion depth and liquid density and its response to the surface tension itself is neglected. The origins and significance of these errors are discussed.