I have thought about this but I can only come up with 5 sources of non-human error. Since we are dealing with gas measurements, we will be using the ideal gas equation PV=nRT, where P is the pressure, V is the volume, n is the number of moles of the gas, R is the ideal gas constant, and T is the temperature. Since R is a constant there is no error associated with it but there can be error in finding either P, V, n, or T. So that is four sources of error. Since we are looking for the molar mass of a gas, that means dividing the mass of the gas (in grams) by the number of moles (n) of the gas. Since this involves weighing the gas on a scale, this is another source of error for a total of 5 sources of non-human error. I cannot think of a sixth source.