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What use is there for a fancy data structure? In practice, these occasions aren't that common. Many "fancy" data structures tend to exhibit bad cache behaviors if implemented naively.
At scale, the law of large numbers says that even improbable events will occur every now and then. Unfortunately, a program with even one bug is still incorrect.
Are you an undergraduate? Depending on how you interpret the spec (which isn't cut and dried when business requirements meet the real world) almost every page of production code has some kind of bug in it. Also, the law of large numbers isn't that relevant for most codebases and developer populations -- the numbers aren't that large. The effect of hubris is much larger in practice.