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."
True story Once upon a time there was a bug that really did depend
on the phase of the moon. There is 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.