[My apologies if this is well-trodden ground. I am pretty ignorant of the ought-implies-can literature, and quick check over articles didn't reveal anybody discussing what I'm about to write.]
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.