Phase of the Moon

Definitions

1. The canonical example of an absurd, uncontrollable variable invoked when a bug's real cause cannot be found. To say a program's behaviour 'depends on phase of the moon' is to admit the failure is non-deterministic and resists every reasonable hypothesis. The phrase carries a famous sting: there are documented cases where it was literally true, most notably a program whose output varied with a timestamp field that encoded the actual lunar phase, and MIT lore about a system whose behaviour changed with building vibration. It survives as the standard sarcastic answer to 'why did it break?' when the honest answer is that nobody knows yet.