[AJTSheppard]

That's a clever point. But I think a corner case.

I suspect that when the user is loading coins or dice in the machine, they would notice any dirt that was significant enough to look as though it might be a problem.

And oil deposits from your fingerprints I would imagine are so minuscule as to be insignificant in creating varying bias.

Even then, in both cases, you could wipe the objects with an alcohol swab before putting them into the shaker cups.

It could be argued, I suppose, that every micro-collision of the coin or die with the cup removes a few atoms, but I would suggest that its effect on the bias of the coin or die over time is again minuscule. Indeed, unmeasurable over a full sequence of cycles (128 for example) of the machine when generating a Bitcoin key.

But an interesting point. Keep 'em coming!