Source : Free On-Line Dictionary of Computing

hidden flag
     
        (scientific computation) An extra option added to a routine
        without changing the calling sequence.  For example, instead
        of adding an explicit input variable to instruct a routine to
        give extra diagnostic output, the programmer might just add a
        test for some otherwise meaningless feature of the existing
        inputs, such as a negative mass.  The use of hidden flags can
        make a program very hard to debug and understand, but is all
        too common wherever programs are hacked in a hurry.
     
        [{Jargon File}]
     
        (1994-11-24)