In two similar and unprecedented case histories during recent earthquakes in Hokkaido Japan, liquefied sand strangely flowed underground in gentle man-made fill slopes of a few percent gradient, leaving large surface depression behind. In both of them, a large amount of non-plastic fines was involved in loose fine fill sands. That particular sand with fines content Fc≈35% tested in undrained triaxial tests was found far more contractive with strain-softening and easier to flow than that of the same density deprived of fines. This strongly suggests that high fines content was the major cause of the strange flow failures because it destined the sand flowable on the contractive side of Steady State Line under sustained shear stress. Another series of cyclic simple shear tests on contractive sands with non-plastic fines under initial shear stress indicated that flow failure tends to occur in gentler slopes when the effective stress path comes across a yield line uniquely drawn from the origin on τ ~ σc ’ diagram irrespective of stress paths. Thus, a scenario to realize the unprecedented flow failures has been clarified based on the field observations and test results.