That's Unheard Of:

Are you familiar with: “This is the sort of English up with which I will not put.” If you are, you've enjoyed the pendantry of trying to "fix" a sentence with a trailing preposition. (By the way, the original may not have been coined by Churchill, and the original form is uncertain, as discussed in a web page called Churchill on Prepositions). Also, I believe pedants will agree that 'will' is correct in the above epigram, not 'shall'. Anyway, here's a sentence that might be fascinatingly hard to fix:
That's unheard of.

Let's start with the Yoda-ese:
Unheard of, is that.

Obviously we've made no improvement. The "of" governs nothing and still hangs at the end of a clause. Our next attempt should get you thrown out of any Writer's Seminar:
Of that, is unheard.

Now here's a real fix, but we have to abandon "unheard":
Of that, nothing's been heard.

The crux of the matter, I believe, is that "unheard" is an unusual word. It doesn't mean quite the same as "not heard". It can be used in some, but not all of the contexts where "not heard" works. "Unheard" is just one step up from an unword.