Greater blue-ringed octopus (photo by Jens Petersen)
An interesting tidbit is that the markings of the Greater blue-ringed octopus (Hapalochlaena lunulata) can be used to identify different individuals. This photo compares a few octopuses, the differences marked with white arrows.
While doing some more reading I found that the blue-ringed ones aren't alone; the "wunderpus" octopus (Wunderpus photogenicus, love that name) individuals can also be distinguished by their patterns. And now we come to the point of this longer-than-intended post. On the left here is a photo of a wunderpus, seen from above - this one seems to have quite a lovely smile! ;)
(Source for modified 2nd photo: Huffard CL, Caldwell RL, DeLoach N, Gentry DW, Humann P, B. MacDonald, B. Moore, R. Ross, T. Uno, S. Wong. 2008. Individually Unique Body Color Patterns in Octopus (Wunderpus photogenicus) Allow for Photoidentification. PLoS ONE 3(11): e3732.)