We have Amdahl's law that basically states that if your program is 10% sequential you can get a maximum 10x performance boost by parallelizing your application.

Another one is Wadler's law which states that

    In any language design, the total time spent discussing
    a feature in this list is proportional to two raised to
    the power of its position.

        0. Semantics
        1. Syntax
        2. Lexical syntax
        3. Lexical syntax of comments

My question is this: What are the most important (or at least significant / funny but true / sad but true) laws of Computer Science and programming?

I want named laws, and not random theorems, So an answer should look something like

Surname's (law|theorem|conjecture|corollary...)

Please state the law in your answer, and not only a link.

Edit: The name of the law does not need to contain it's inventors surname. But I do want to know who stated (and perhaps proved) the law