G. A. Cohen has a diagnosis of what he thinks is going wrong with political philosophy, a diagnosis that is supposed to explain why, say, Rawls, Dworkin, and Nagel, or Nozick, Schmidtz, and Gaus, are relatively sanguine about markets, while Cohen thinks markets are inherently repugnant. The problem in his view is that philosophers illicitly allow facts about people are willing to do to act as a constraint on what they ought to do. But while it may be the case that you can't have a duty to X unless you can X, that doesn't mean that you can't have a duty to X just because you aren't willing to do X.
In general, philosophers take it for granted that a person morally ought to X if and only she can X. I'll treat this as equivalent to claiming that a person can have an all-things-considered moral duty to X if and only she can X. So, e.g, I cannot have an all-things-considered duty to shoot magic fireballs from my fingers because I am physically unable to do so.
Outside of ethics, though, there seem to be a few cases where intuitively we don't accept this, or, at least, I don't.
Five years ago I was up-to-date on virtue ethics. I've been ignoring it for a few years, so please forgive me if what I'm about to say is old news.
Rosalind Hursthouse argues: "An action is right iff it is what a virtuous agent would characteristically do in the circumstances, except for in tragic dilemmas, in which a decision is right if it is what the agent would choose, but the action decided upon may be too terrible to be deemed right or good."
One traditional worry with this is a variation of the Euthyphro problem: Does the virtuous agent perform the action because it's right or is it right because the virtuous agent would do it. Hursthouse takes the first horn of the dilemma and tries to argue that this is not a problem for virtue ethics.
However, I think that the analysis of right action is mistaken. There are (I think) counterexamples.
First, thanks to Doug and the other editors for the invitation! I'm happy to be here.