A little creature with big eyes took the opportunity to get in front of the camera of the mars rover just at the moment the front Hazcam of the rover took a picture of the surrounding area.
It may just be the imagined perception of a pattern what actually does not exist (paradoleila) but in my opinion you can clearly see a very small animal (head, left big eye, tongue, legs and tail) that lies on the ground.
Anyhow, take a look at this picture and be ready to be amazed.
Opportunity Mars Rover: SOL 4219
Link image: http://mars.nasa.gov/mer/gallery/all/1/f/4219/1F502741534EDNCOPRP1141R0M1.JPG