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.