I would accept mistakes and inconsistency from a human, especially one not very experienced or skilled. But I expect perfection and consistency from a machine. When I command my computer to do something, I expect it to do it correctly, the same way every time, to convert a particular input to an exact particular output, every time. I don't expect it to guess, or randomly insert garbage, or behave non-deterministically. Those things are called defects(bugs) and I'd want them to be fixed.