[AndresNavarro]

Adding the numbers is not the problem, the problem is what the numbers represent, where do they come from (if they come from only one place...) where and for what are they needed.

Quickly you have to consider different kinds of numeral systems, bases, decimal separators (which may be locale dependant), precision, units, the difference in adding time (or dates), or money in different currencies (with and without different kind of taxes), etc, etc, etc.

Sure most of the times you can abstract yourself from all this, use sensible defaults, whatever, but that complexity is still there and sooner or later it will get you, even if you try to hide it under the rug...

[_y5hn]

Exactly this!

The complexity and misunderstandings also creep into system design itself. Seems many are eternally astounded that transactional data is "duplicate data" stored separately from the "same data", etc. Many of these complexities are non-obvious and unintuitive, until you are forced to think it through step by step yourself. Agile is simply the concession that the complexity cannot always be handled up-front, but that solutions must be developed to be adaptable to new observations and realizations.