phase of the moon

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 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.