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.

