Any adequate axiomatisable theory is incomplete. In particular the sentence "This sentence is not provable" is true but not provable in the theory.
Gödel's Second Incompleteness Theorem
In any consistent axiomatisable theory (axiomatisable means the axioms can be computably generated) which can encode sequences of numbers (and thus the syntactic notions of "formula", "sentence", "proof") the consistency of the system is not provable in the system.
It always takes longer than you expect,
even when you take into account Hofstadter's Law.
— Douglas Hofstadter 'Gödel, Escher, Bach: An Eternal Golden Braid'
A file that big? It might be very useful. But now it is gone.
Windows XP crashed. I am the blue screen of death. No one hears your screams. First snow, then silence. This thousand dollar screen dies So beautifully.
With searching comes loss And the presence of absence: "My novel" not found.
A crash reduces Your expensive computer To a simple stone.
Three things are certain: Death, taxes, and lost data. Guess which has occurred.
You step in the stream, But the water has moved on. This page is not here.
Having been erased, The document you're seeking Must now be retyped.
Yet such is oft the course of deeds
that move the wheels of the world:
small hands do them because they must,
while the eyes of the great are elsewhere.