Somebody tells you to obey whatever instructions they give you, no matter what. Assume for the moment that, for some reason -- love, loyalty, respect, life-and-death situation, whatever -- you are willing to go ahead. The first instruction is for you to do something, and keep on doing it even if that person subsequently asks you to stop. You say OK, and proceed to do the thing, upon which he/she immediately cries for mercy and begs you to stop, saying that the thing you are doing is too painful. What do you do?
The obvious answer is to persist, because didn't the person ask you not to stop, no matter what? Not so fast. If you have go by the meta-instruction -- "Obey whatever instruction I give you" -- what prevents you from obeying the second instruction?
I don't think there is a logically sound answer to this [*]. I am at home on sick leave, so I don't have the energy to think this through, but things like self-referentiality, meta-levels, and the fact that it is impossible to completely answer the question "What is truth?" (maybe Godel's Theorem as well?) come to mind.
(Context: I am about 530 pages or so into Harry Potter and the Half-Blood Prince, the scene where Dumbledore and Harry Potter are attempting to destroy one of Lord Voldemort's Horcruxes. Sheepish grin).
[*] On second thoughts, there is a logically sound answer -- keep obeying the latest instruction till the requester drops dead or at least loses the ability to issue further instructions -- but this doesn't really serve the purpose.