Yeah, now that I think about it, Anakin doesn't count because he is a deus ex machina in the narrative, not as a role he does in the films. A better one I just thought of is Donald Pleasence in the original Halloween; he saves Laurie at the end but Michael still gets away.
But that's the whole crux of the question though isn't it? Can you be a deus ex machina if you don't do the thing you are supposed to do? The fact that you failed at the deus exing, would imply that you AREN'T a deus ex machina, because you didn't fix the mess by stint of being there. So, I personally don't think it's possible to be a failed deus ex, not as the descriptive term for the narrative tool. I think we're conflating Chosen One with Deus Ex in this case perhaps? As you can often how a False Chosen, and Anakin is a prime example. But Chosen One is more of an in-universe thing, describing the expectations of a populace upon an individual. Whereas Deus Ex is more an external-universe label, for a plot device and what it accomplishes, given a narrative structure.
Again, that's just my take on the terminology we're tossing about, that no, by definition, it's impossible to have a Deus Ex Machina that doesn't accomplish the deus exing, as if they didn't do it, they wouldn't be a Deus Ex Machina. It's like calling someone the winner of some competition, when they actually lost.
