Upon first glance, one might find the sentence "More people have been to Berlin than I have" to be perfectly acceptable. If you look a little closer, you'll find the sentence has no meaning whatsoever. This type of sentence is called a comparative illusion or an Escher sentence. If you're still scratching your head, unsure of why the sentence sounds so strange, here's why:
"Escher sentences are ungrammatical because a matrix clause subject like more people is making a comparison between two sets of individuals, but there is no such set of individuals in the second clause. For the sentence to be grammatical, the subject of the second clause must be a bare plural. Linguists have marked that it is 'striking' that, despite the grammar of these sentences not possibly having a meaningful interpretation, people so often report that they sound acceptable, and that it is 'remarkable' that people seldom notice any error (Comparative illusion explained)".
