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A “phase-of-the-moon bug” is a bug that is dependent on some external factor, often a factor that is actually undetermined. The phase-of-the-moon bug is also sometimes called a “Heisenbug” for Heisenberg's uncertainty principle, when the bug is inconsistent or unable to be replicated on return attempts.
Phase-of-the-moon bugs can be extremely difficult to identify and fix. Changing conditions may cause the bug to surface, but may not allow for it to be replicated later. This may lead to long debugging sessions as frantic engineers scramble to find the cause of the phase-of-the-moon bug. People use the phrase “phase of the moon” to describe the quirkiness of the bug in question or its elusive refusal to follow certain rules. Some suggest that disciplines such as unit testing can reduce or eliminate the chances of developing phase-of-the-moon bugs.