How Do You Refactor Untested Code?
“Bug fixing often uncovers opportunities for refactoring. The very fact that you’re working with code that contains a bug indicates that there is a chance that it could be clearer or better structured.”
Articles by Software Testing Magazine
About the Existence of Bugs
Program testing can be used to show the presence of bugs, but never to show their absence! Edsger W. Dijkstra Source: Notes On Structured Programming, 1970, at the end of section 3, On The Reliability of Mechanisms.