Recent studies report an anomalous phenomenon, particularly for small diameter microfin tubes, where the flow boiling heat transfer coefficient decreases with increasing mass velocity in the convective regime, which is contrary to that found for smooth tubes and larger diameter microfin tubes. This paper presents a critical literature review and mechanistic analysis of the anomalous phenomenon. Our analysis suggests that the anomalous phenomenon is a result of the transition of annular flow pattern from “flooded-groove” (film thickness > fin height) to “in-groove” (film thickness < fin height). The latter is associated with lower degree of turbulence, smaller wetted area, and therefore smaller heat transfer coefficient than the former. We speculated that the in-groove annular flow only occurs for small diameter tubes where the liquid film is relatively uniform, while larger tubes tend to remain flooded at the tube-bottom until dryout. This may explain the tendency of the anomalous phenomenon to occur in small diameter tubes.