Source: http://sploid.gizmodo.com/did-a-martian-throw-a-rock-at-a-mars-opportunity-rover-1503335556/@barrett
Left: a photo taken 3528 days after the Opportunity rover arrival to Mars. Right: the exact same spot 12 Mars days later. Notice the difference? NASA JPL scientists did too: "It's about the size of a jelly doughnut. It was a total surprise, we were like 'wait a second, that wasn't there before, it can't be right. Oh my god! It wasn't there before!' We were absolutely startled."