A good threat is worth a thousand tests.
In programming, it’s often the buts in the specification that kill you.
A test that reveals a bug has succeeded, not failed.
More than the act of testing, the act of designing tests is one of the best bug preventers known.
If you can't test it, don't build it. If you don't test it, rip it out.
If the objective of testing were to prove that a program is free of bugs, then not only would testing be practically impossible, but it would also be theoretically impossible.