Orifice flow meters are among the most widely used devices in industrial measurements. Their popularity stems primarily from their versatility across various fluid flows, high accuracy, and broad measurement range. However, these devices demand strict installation conditions to ensure accurate measurements, which can be challenging, if not difficult, to meet in industrial environments. Measurements conducted on the research stand for two-phase water-air flow allowed for the estimation of discrepancies between flow rates determined by a reference electromagnetic flow meter (measuring water volumetric flow) and an orifice flow meter (measuring two-phase volumetric flow). The observable decrease in the pressure drop (Δp) indication at the ISA orifice plate with flange taps suggests the presence of errors in volumetric flow rate determination during two-phase flow measurements. Furthermore, the presence of an additional phase increases measurement uncertainty, leading to acquired data being subject to significant errors.