The fence built on the hill's slop over time will fall. It will not be a sudden process though, but slowly, the fence will bend over till it falls. This will be caused by the erosion, over time little by little the hill will be eroding because of the natural phenomenons like the wind and the rain (depending on the place where the hill is, maybe ice as well), so there will come a point where the basis of the fence that were once in the ground will be uncovered and there will be nothing to hold it, so it will fall.
