ThevalidityofNavier'spartialslipconditionisinvestigatedbystudyingtheoscillatoryflowinacoatedchannel.Thetwo-fluidmodelisusedtosolvetheunsteadyviscousequationsexactly.Partialslipisexperiencedbythecorefluid.ItisfoundthatNaviersconditiondoesnotholdforanunsteadycorefluid.