I see exchanges like this in stories all the time:
“You'll have to come down to the station with us,” the police officer said.
“You don't understand,” said the mysterious child with the beatific smile. “I can't.”
Generally, that back-and-forth is repeated three or four times, with the first character (the officer in the above example) getting more and more annoyed, or providing more and more incentive, and the second character (the kid in the above) continuing to repeat the same thing.
And then eventually (usually much later in the story) it turns out that the kid is speaking the literal truth. Maybe they're a ghost, bound to that one location. Maybe if they leave, the world will end. For whatever reason, it's really not that the kid doesn't want to go, it's that they can't, just like they said.
But you know what would save a lot of time, not to mention the patience of all readers who've seen this in stories before?
If the kid would actually explain why they can't. Instead of just saying they can't over and over.
Some authors try to finesse this by having the “can't do that” character attempt to explain, only to be cut off by the impatient non-understanding character. That's a slight improvement, but I still find it kind of tedious to read; all the back-and-forth is just filler, because I know it's going to turn out that the person really does mean “can't.”
(It's a little like all those stories in which a bad person is told they'll receive “what you deserve” and doesn't realize that means they're going to get something bad.)
The non-understanding character almost never seems to notice the repetition. You'd think even if the “can't do that” character is refusing to follow conversational norms by elaborating on the “I can't” statement, the other person might say, wait, you keep saying that, so I'm clearly interpreting your comment differently from what you meant; what exactly do you mean by “I can't”? And then the “can't do that” character could explain.
(Well, okay, once in a while the non-understanding character does say “What do you mean?”, but in those cases the “can't do that” character almost always shrugs sullenly and says, “I just can't.” Because saying the three to five words of explanation that would clear the whole thing up would be giving away the surprise that the author thinks the reader hasn't picked up on yet.)
There are occasionally cases where this is handled reasonably well: for example, if what the character is incapable of doing is speaking about certain things, or if the character who hears the “I can't” does figure out that it really means can't rather than won't.
But it still tends to push my buttons a bit. Mostly because it steps on another of my pet peeves: characters who know what's going on but refuse to tell the protagonist, which generally seems contrived to me, designed to keep the reader in artificially constructed suspense. But also partly because I am a literalist, so when I hear or read the word “can't,” my first assumption is that it really means “can't”; I rarely hear people say “I can't do that” in real life when what they really mean is “I'm arbitrarily saying no to your request or demand because I feel like it.” So I find it frustrating when fictional characters assume that “I can't” means the latter rather than the former.
(The parallel with “get what you deserve” is fairly strong in that regard: I almost never encounter the phrase “get what you deserve” or “get what's coming to you” in a positive context, so it seems weird to me that villains, even megalomaniacal ones, would interpret it to mean “I'll get good things.”)
(Wrote this in April of 2010, apparently neglected to post it.)