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Computing (FOLDOC) dictionary (also found in English - English (Wordnet), )
phase of the moon
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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.