Theoretically, you can write ideal program, without any bugs and with excellent usability. The practice guarantee that someone from the human race will break it someday.
The greatest test plan is very handy for the testers to eliminate errors during testing run of the program. But you can be sure that tester will make a mistake eventually and will leave the bug in the production code.
The program can be perfectly designed, and finally it will be broken by the human. Moreover, the piece of software can be bad and ugly, and crash always, and some humanoid will find a way to crash it even worse.
Human can write a lots of automatic tests for program's safety network. He will miss something, and the bug will live in the program. The bug will be discovered, new test will be added, and the infinite set of human errors will be decreased by one.
Definitely, we shall prevent human race from interacting with anything related to the software. Don't touch that keyboard!
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So if you want a perfect
So if you want a perfect program, don't even think about writing it, imperfect human programmer!
The keyboard!!!
You have touched it! And made an input. Potentially, you have broken a lots of things.
Definitely, the human factor shall be eliminated :)