We spend a lot of time debating what non standard behaviours are safe to
rely on because other software relies on it. Someone actually went and
surveyed some major projects about what behaviours are being counted on.
Three results aren't as conclusive as I would have expected but even so
some of then are still interesting.
http://www.cl.cam.ac.uk/~pes20/cerberus/notes50-2015-05-24-survey-discussion.html