Two phenomena for flows in microchannels, namely visco-elastic instability and Faradaic charging effects, are discussed. Aqueous solutions, which do not exhibit elastic behaviors, will behave visco-elastically with the addition of a small amount of polymers. Visco-elastic flow instability which promote mixing has been observed in the flow of these visco-elastic solutions through a constriction in microchannels. In addition, we recently show experimentally the existance of Faradaic charging in AC-electrokinetic (ACEK) flow. Lastly, surface roughness effects in micro- plastic injection molding are highlighted. The micro-injection molding process involves the flow of polymer melt in micro-cavities. Our recent investigations demonstrate that the flow model for macro-cavities could be employed satisfactorily to simulate the flow in micro-cavities if roughness effects are considered in an appropriate manner.