Two Wrongs Can Make a Right (and Are Difficult to Fix)

Code never lies, but it can contradict itself. Some contradictions lead to those "How can that possibly work?" moments.

In an interview, the principal designer of the Apollo 11 Lunar Module software, Allan Klumpp, disclosed that the software controlling the engines contained a bug that should have made the lander unstable. However, another bug compensated for the first and the software was used for both Apollo 11 and 12 Moon landings before either bug was found or fixed.

Consider a function that returns a completion status. Imagine that it returns false when it should return true. Now imagine the calling function neglects to check the return value. Everything works fine until one day someone notices the missing check and inserts it.

Or consider an application that stores state as an XML document. Imagine that one of the nodes in incorrectly written as TimeToLive instead of TimeToDie, as the documentation says it should. Everything appears fine while the writer code and the reader code both contain the same error. But fix one, or add a new application reading the same document, and the symmetry is broken, as well as the code.

When two defects in the code create one visible fault, the methodical approach to fixing faults can itself break down. The developer gets a bug report, finds the defect, fixes it, and retests. The reported fault still occurs, however, because a second defect is at work. So the first fix is removed, the code inspected until the second underlying defect is found, and a fix applied for that. But the first defect has returned, the reported fault is still seen, and so the second fix is rolled back. The process repeats but now the developer has dismissed two possible fixes and is looking to make a third that will never work.

The interplay between two code defects that appear as one visible fault not only makes it hard to fix the problem but leads developers down blind alleys, only to find they tried the right answers early on.

This doesn't happen only in code: The problem also exists in written requirements documents. And it can spread, virally, from one place to another. An error in the code compensates for an error in the written description.

It can spread to people too: Users learn that when the application says Left it means Right, so they adjust their behavior accordingly. They even pass it on to new users: "Remember when that applications says click the left button it really means the button on the right." Fix the bug and suddenly the users need retraining.

Single wrongs can be easy to spot and easy to fix. It is the problems with multiple causes, needing multiple changes, that are harder to resolve. In part it is because easy problems are so easily fixed that people tend to fix them relatively quickly and store up the more difficult problems for a later date.

There is no simple advice to give on how to address faults arising from sympathetic defects. Awareness of the possibility, a clear head, and a willingness to consider all possibilities are needed.