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Two, and only two?
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Well:

    There are actually only two hard problems in computer science:
        0) Cache invalidation
        1) Naming things
        4) Asynchronous callbacks
        2) Off-by-one errors
        3) Scope creep
        6) Bounds checking
* https://news.ycombinator.com/item?id=38806257

Also: https://martinfowler.com/bliki/TwoHardThings.html

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You have almost understood the joke.
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two is off-by-one from three btw
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