Programs produced by
evolutionary techniques such as
Genetic Algorithms. The elements of the language are shuffled in a
pseudo-random way, that creates only feasible or working variants.
Although similar to unreadable code it holds the distinction of being also fatter and more roundabout. Since the code is evolved only for its ability to solve a problem - and not the length or readability - it can be even more obscure than human written programs.
Even worse is evolvable hardware, which can sometimes use physical effects like quantum tunneling and can therefore be sensitive to temperature changes and features specific to the particular model.