"It" is the vague pronoun. The referent in the sentence is not clear. It is difficult to determine whether this pronoun refers to the car or the wall. As a result, the sentence is ambiguous and can be interpreted in two ways, one is that the car moved backwards but it was not damaged(while the wall might be damaged); the other interpretation is that the wall was not damaged even though the car hit it(maybe the car ended up with marks).