The assumption of low Reynolds number flow, or Stokes flow, is often applied to the understanding of a broad range of microfluidic devices, including micro-reactors, biomedical devices, and membraneless electrochemical cells. However, recent studies have shown that various inertial effects can play a significant role, even in microfluidic systems. In this work, two- and three-dimensional secondary flows are identified in a generic rectangular flow channel design consisting of a secondary channel feeding fluid into a main channel. We identify a scaling argument which is able to predict the occurrence of these secondary flows as a function of system parameters. The impact of these behaviours on the assumption of fully developed colaminar flow is investigated. This work considers a representative geometry, and identifies a set of conditions where inertial effects can play a key role in a microfluidic device.