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optimism: n.

What a programmer is full of after fixing the last bug and before discovering the next last bug. Fred Brooks's book The Mythical Man-Month (See Brooks's Law) contains the following paragraph that describes this extremely well:

All programmers are optimists. Perhaps this modern sorcery especially attracts those who believe in happy endings and fairy godmothers. Perhaps the hundreds of nitty frustrations drive away all but those who habitually focus on the end goal. Perhaps it is merely that computers are young, programmers are younger, and the young are always optimists. But however the selection process works, the result is indisputable: “This time it will surely run,” or “I just found the last bug.”.

See also Lubarsky's Law of Cybernetic Entomology.