- Where There Is One Bug, There Is Likely to Be Another

This is a restatement of the principle that states when you find an error in a section of a program, the probability of the existence of another error in that same section is higher than if you had not already found one error. In other words, errors tend to cluster. When repairing an error, examine its immediate vicinity for anything else that looks suspicious.

- Fix the Error, Not Just a Symptom of It

Another common failing is repairing the symptoms of the error, or just one instance of the error, rather than the error itself. If the proposed correction does not match all the clues about the error, you may be fixing only a part of the error.

- The Probability of the Fix Being Correct Is Not 100 Percent

Tell this to someone and, of course, he would agree, but tell it to someone in the process of correcting an error and you may get a different answer.