Sometimes, I'll be coding away, make a change that I somewhat expect to break things, and nothing breaks.
Inwardly, I cheer. My test suite is perfect! It has allowed me to refactor with abandon. However, this quickly passes, and I feel the need to know if I am doing it "right", that is, I trust my intuition that it should be broken. Am I running the right version of the code? Did I restart the server to pickup my changes?
Often, I'll find that I was running an older, known-good bit of code, and that my change wasn't picked up. Once I force my changes to the local test server, I can see my changes have subtly broken something, and resume my fixing.
I like to think of this of healthy paranoia. If I've changed something that should've broken at least something, I should be looking for breakage. If none occurs, likelihood is that I've done something wrong (aside from writing bad code).