That is not an off-by-one error in a computer science sense, nor is it "one of the hardest problems in computer science".
This was in reference to a well-known joke, see here: https://martinfowler.com/bliki/TwoHardThings.html
This was in reference to a well-known joke, see here: https://martinfowler.com/bliki/TwoHardThings.html