A moral pardox first posed by William Glass.
Imagine it this way: You ask someone if you can use her in an experiment. She says yes, and so you knock her out and put her in the oven. The phone rings, you lose track of time, and your experiment is ruined.
The paradox is this: one does not, as commonly supposed, use morality to tell whether the above action is morally correct or not. Instead, it works the other way around: one uses whether a system of ethics disallows the above action to judge the system. If you have a set of ethics which allows baking the obliging stranger, I will feel justified in ignoring it no matter how sound it may be.
For Glass, this opens the question: Is there an absolute set of ethics? But if so, how can we ever deduce what that set is?