The Law of Conservation of Complexity on Wikipedia
This law states that there is a certain amount of complexity in a system which cannot be reduced.
Some complexity in a system is 'inadvertent'. It is a consequence of poor structure, mistakes, or just bad modeling of a problem to solve. Inadvertent complexity can be reduced (or eliminated). However, some complexity is 'intrinsic' as a consequence of the complexity inherent in the problem being solved. This complexity can be moved, but not eliminated.
One interesting element to this law is the suggestion that even by simplifying the entire system, the intrinsic complexity is not reduced, it is moved to the user, who must behave in a more complex way.