CODE NEVER LIES, BUT IT CAN CONTRADICT ITSELF.
Consider a function that returns a completion status. Imagine that it returns false when it should return true. Now imagine that the calling function neglects to check the return value. Everything works fine until one day someone notices the missing check and inserts it.
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 for how to address faults arising from sympathetic defects. Awareness of the possibility, a clear head, and a willingness to consider all possibilities are needed.