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The thing which is not.

In most mathematical logic systems "false" or "⊥" has the nice property that anything can be derived from it (⊥ -> p, for any sentence p). I find it funny that the undefined behavior of null dereferencing works the same way in C - literally anything can happen to the program, so the compiler is free to assume (derive about the current state of execution) anything it wants.

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It's not so much "funny" as a rather obvious consequence of the principal

Anything can be derived from p or not p, not from false.

This is just a corollary of the fact that "p or not p" is false (for any value of p).

"p and not p" is false, "p or not p" is always true.

Oops. Went a bit on autopilot with that one.

Then everything is true. :)

The sentence "p or not p" is a tautology ("true", ⊤) in classical logic, so you can't derive anything useful from it. It can carry some power in non-classical logics, like the intuitionistic logic, where it's not true that "p or not p" for any sentence p.

You can derive anything from false, this is known as the principle of explosion: "ex falso quodlibet", "from false anything":

https://en.wikipedia.org/wiki/Principle_of_explosion

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