How programmers make sure that their software is correct

What the programmer wants the software to do and the needs of the user are not necessarily the same. In fact, depending on the organization, there may be a significant disconnect between the two. Being in a medical industry, I have to constantly deal with the difference between verification and validation. In other words, a piece of software functionality I implemented may do exactly what I want it to. It may fulfill all stated requirements. And yet it may still fail a round of validation because it does not result in a usable product.

(Edited spelling/mobile typos)