I spent a day debugging a problem this created. Without going into irrelevant domain details, we had a set of objects, each of which has numeric properties A and B. The formal specification says objects are categorized in a certain way iff the sum of A and B is less than or equal to 0.3.

The root problem, of course, was that 0.2 + 0.1 <= 0.3 isn't actually true in floating point arithmetic.

It wasn't immediately obvious where the problem was since there were a lot of moving parts in play, and it did not immediately occur to us to doubt the seemingly simple arithmetic.