Reports yesterday of a lock screen bypass in the iPhone 5 noted that a "similar" bug was found in iOS 4.1 and fixed in 4.2. In both cases, the lock screen, which is only supposed to let you make emergency calls or enter the lock code, allows the user to perform other functions, like make other phone calls. How do these errors resurface after being fixed? In Apple's case, the problem could be a weakness in their test plans or procedures.

When an error that was fixed shows up again later it is called a regression error. Regression errors generally are when some change to the program, a new version or software patch, breaks some feature of the program. Security fixes are one type of feature that could be broken.

Controlling regression errors is a matter of proper documentation and testing. Good code documentation should at least give future developers the chance to recognize that changes will affect the feature. But it's testing that is the key to preventing regressions.

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