I was just mentioning linguistic recursion in another thread, and this is closely related to the concept of unique sentences. Basically, every language has a finite set of words and rules but they yield an infinite number of valid sentences.
It was actually a little private joke between my ex and I. Sometimes, one of us would utter a weird, yet perfectly meaningful sentence. We would then pause and then go : “Pretty sure nobody has ever said this”. Unfortunately, I can’t remember any specific examples now…