This bug causes Ue4 to chew up all available RAM before seizing up the computer (due to no memory). This was tested on a system with 128GB ram and a 128GB paging file with nothing else open. UE4 managed to eat all of the memory! It seems to be caused by a spline segment shooting off into infinity.
Steps to reproduce (save all work first!)
1: Create a landscape
2: Create a landscape spline and apply a simple mesh
3: Select a segment and offset on the Y or X axes, this sometimes takes a few tries but usually the second or third try UE4 will hide your cursor and seize up, it will then proceed to
2 suggested fixes are sorting out the sensitivity to 1 pixel does not = 100km!!! (exponential increase cap) or putting a distance cap on it.