Sometimes used to refer to a bug in a computer program which is only reproducible sometimes, especially if in an erratic fashion.

Allegedly originates from the following incident:


There's the famous phase-of-the-moon bug which struck (I believe) Gerry Sussman and Guy Steele, then both of MIT. It turned out to be due to code which wrote a comment into a file of LISP forms, that included the phase of the moon as part of the text. At certain times of the month, it would fail, due to the comment line being longer than the "page width"; they had failed to turn off automatic newlines that were being generated by Maclisp when the page width was exceeded. Thus the last part of the line would be broken onto a new line, not proceeded with a ";" (the comment character). When reading the file back in, an error would result.