I've found a couple of references which give the following as "Brooke's Law":
"Whenever a system becomes completely defined, some damn fool discovers something which either abolishes the system or expands it beyond recognition."
In other words, nothing's ever finished. By the time you've fully solved the problem you set out to solve, one of two things will have have happened: Either the problem itself will have changed, or your initial knowledge of the problem will prove to have been faulty or incomplete.
This is one of the many, many crosses that software developers bear. On the other hand, we get to play Quake at work. I'm not complaining.
Brooks's Law was stated by Frederic P. Brooks, and it's something else entirely.