There are rumors going around that the asteroid that may (but probably won't; the odds recently dropped slightly) smack into Mars on January 30 might be the lost Mars Observer, a NASA probe that malfunctioned just as it was to enter Mars orbit. I strongly doubt this to be the case; the orbit is wrong for a Mars probe; tracing backwards, the orbit hasn't come close to Earth for many decades.
It's tempting, though; the size of an object is judged by how bright it is, how far it is, and an assumed reflectivity. A rock is dark, while ice is bright; so for an object at a certain distance and brightness, it would be bigger if it were made of rock than if it were made of ice (you need more surface area of rock to make it as bright as a smaller object made of ice).
Metal is shiny, so a smallish spacecraft would look like a much bigger rock when observed from Earth. Still, I really think we have just a rock here. And I really don't think it'll hit.
But if it does...