One great poem I found:

Run your tests often,

Don't let them get stale.

Rejoice when they pass.

Rejoice when they fail.

^^I don't have the source of the original author, I wish I did. At this point in my career, I don't care

• how haphazard the codebase may be
  
• how brittle the architecture stands
  
• managed/unmanaged code of any and all languages
  

Well-covered tests prove that the system is robust, end of story.