Opportunity landed on Mars in January 2004 with an anticipated 3 month mission lifetime. Ten years later it is still roving, and exploring the Martian surface.
The second of the two images shown, in which the rock appears, was taken about 12 Martian days (each of 24 hours 39 mins) after the first. At first I thought this was a hoax, but the truth of the matter is revealed on NASA’s Opportunity website:
The ground operations team were genuinely surprised by the ‘sudden’ appearance of the rock, where there was none beforehand. Possible theories being suggested were that the rover may have flipped the rock into its new position as it passed by, or that it had just landed there after a meteorite impact nearby. Clearly there were other
ideas such as little green men (or Elvis?) throwing stones at the intruder. Whatever the cause, NASA scientists now consider the rock worthy of further scientific scrutiny.