From Leopard Gecko Wiki
A Paradox is a random occurence of something that shouldnt happen on a Leo. For example, All albino leopard geckos are amelanistic (cant produce balck pigment), so an albino with a black spot on would be a paradox. Or a Patternless leopard gecko with some spots.
Also, animals with unusual black or red markings are named as paradox. Those spots can occur everywhere on the body, for example on legs & belly.