Viking 1 landing site. Upper image: this site shows a variety of rock sizes, most of them harmless at the scale and clearance of the Lander. However, it might just take one...Only few yards from the lander, in the lower image, Big Joe --a boulder of metric-scale-- would have proved fatal to the mission if the lander had arrived on top of it. It would have most likely stumbled on it.
Source: NASA, Viking Mission