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

dancing frog
 n.

    [Vancouver area] A problem that occurs on a computer that will not reappear
    while anyone else is watching. From the classic Warner Brothers cartoon One
    Froggy Evening, featuring a dancing and singing Michigan J. Frog that just
    croaks when anyone else is around (now the WB network mascot).


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

dancing frog

    A bug or glitch that only occurs for
   a particular user; never when the user tries to show it to
   anyone else.

   The term is derived from a Warner Brothers cartoon in which a
   man discovers a frog which can sing and dance; he believes
   this will make his fortune but the frog never performs in
   front of anyone else.

   (2004-10-16)