Free will.
An essential part of every significant program to prevent it from becoming too deterministic and predictable. An element of randomness in an unrandom world. See also Heisenberg's Programming Principle:
The act of fixing the last bug will in all probability result in more.
The first bug was a moth inside a thinking machine - an element of free will that gave the designers something to do. Thus began the long and glorious history of bug fixing - the never-ending quest to eliminate all free will from all computer programs, the quest for an unreachable perfection.
The next time you're faced with a mountain of bugs on your plate, relax and remember that perfection is a process. It's not meant to be reached. There will always be another bug, another mission. In between, remember every once in a while to take a break and smell the java. Your life will appreciate it.
The rest of the computer may be deterministic, but your mind is not. That's the beauty of it all.
From the as yet unwritten
Zen and the Art of Computer Programming