hidden flag

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.