This article summarizes some mistaken ideas of diffusion and effusion in current chemistry textbooks. There are only two conditions where Graham's law apply in diffusion experiments: (a) diffusion across isobaric porous barrier (b) Effusion through a hole to vacuum. The standard mutual diffusion experiment of NH3 versus HCl does not satisfy the condition of Graham's law. Finally, we make a comparison concerning diffusion in the new editions of Taiwan's high school textbooks.