[1] : Jargon File (4.3.1, 29 Jun 2001)

hidden flag n. 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 on in a
   hurry.

[2] : The Free On-line Dictionary of Computing (27 SEP 03)

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
     
     
see also:
jargon file 

Flag >> F Words
Flag, definition of term: Flag
flag_pag1.html

Flag Stop >> F Words
Flag Stop, definition of term: Flag Stop
flag+stop_pag1.html

Flag Officer >> F Words
Flag Officer, definition of term: Flag Officer
flag+officer_pag1.html

Flag Day >> F Words
Flag Day, definition of term: Flag Day
flag+day_pag1.html

Black Flag >> B Words
Black Flag, definition of term: Black Flag
black+flag_pag1.html