WordNet (r) 3.0 (2006):

phase of the moon
    n 1: a time when the Moon presents a particular recurring
         appearance

The Jargon File (version 4.4.7, 29 Dec 2003):

phase of the moon
 n.

    Used humorously as a random parameter on which something is said to depend.
    Sometimes implies unreliability of whatever is dependent, or that
    reliability seems to be dependent on conditions nobody has been able to
    determine. ?This feature depends on having the channel open in mumble mode,
    having the foo switch set, and on the phase of the moon.? See also 
    heisenbug.

    True story: Once upon a time there was a program bug that really did depend
    on the phase of the moon. There was a little subroutine that had
    traditionally been used in various programs at MIT to calculate an
    approximation to the moon's true phase. GLS incorporated this routine into
    a LISP program that, when it wrote out a file, would print a timestamp line
    almost 80 characters long. Very occasionally the first line of the message
    would be too long and would overflow onto the next line, and when the file
    was later read back in the program would barf. The length of the first
    line depended on both the precise date and time and the length of the phase
    specification when the timestamp was printed, and so the bug literally
    depended on the phase of the moon!

    The first paper edition of the Jargon File (Steele-1983) included an
    example of one of the timestamp lines that exhibited this bug, but the
    typesetter ?corrected? it. This has since been described as the
    phase-of-the-moon-bug bug.

    However, beware of assumptions. A few years ago, engineers of CERN
    (European Center for Nuclear Research) were baffled by some errors in
    experiments conducted with the LEP particle accelerator. As the formidable
    amount of data generated by such devices is heavily processed by computers
    before being seen by humans, many people suggested the software was somehow
    sensitive to the phase of the moon. A few desperate engineers discovered
    the truth; the error turned out to be the result of a tiny change in the
    geometry of the 27km circumference ring, physically caused by the
    deformation of the Earth by the passage of the Moon! This story has entered
    physics folklore as a Newtonian vengeance on particle physics and as an
    example of the relevance of the simplest and oldest physical laws to the
    most modern science.


The Free On-line Dictionary of Computing (30 December 2018):

phase of the moon

   Used humorously as a random parameter on which something is
   said to depend.  Sometimes implies unreliability of whatever
   is dependent, or that reliability seems to be dependent on
   conditions nobody has been able to determine.  "This feature
   depends on having the channel open in mumble mode, having the
   foo switch set, and on the phase of the moon."

   See also heisenbug.

   True story: Once upon a time there was a bug that really did
   depend on the phase of the moon.  There was a little
   subroutine that had traditionally been used in various
   programs at MIT to calculate an approximation to the moon's
   true phase.  GLS incorporated this routine into a Lisp
   program that, when it wrote out a file, would print a
   timestamp line almost 80 characters long.  Very occasionally
   the first line of the message would be too long and would
   overflow onto the next line, and when the file was later read
   back in the program would barf.  The length of the first
   line depended on both the precise date and time and the length
   of the phase specification when the timestamp was printed, and
   so the bug literally depended on the phase of the moon!

   The first paper edition of the Jargon File (Steele-1983)
   included an example of one of the timestamp lines that
   exhibited this bug, but the typesetter "corrected" it.  This
   has since been described as the phase-of-the-moon-bug bug.

   [Jargon File]

   (1995-02-22)