If you truly have "no room for error" (i.e. you are shipping code to a hundred-billion-dollar space vehicle), you might have:

- Very strict coding guidelines

- Rigorous code review

- An extremely thorough test suite

- Multi-stage approval through a QA process involving many pairs of eyes and thorough, formally defined and scientifically rigorous test procedures

- Static analysis and possibly even algorithms proved correct in Coq

Simply playing with it in staging for a while and determining that everything looks good is NOT the kind of testing that you do when there's "no room for error."