POSIX_ME_HARDER is an environment variable that can be used as an alias for the POSIXLY_CORRECT variable for some programs, particularly those written by people exasperated by some aspect of the POSIX standards. See Democracy Triumphs in Disk Units.

That page suggests that df(1) uses POSIX_ME_HARDER, but recent versions don't, at least:

[crb@jane]: ~$ df
Filesystem           1k-blocks      Used Available Use% Mounted on
/dev/hda3             18690576   3958640  13782496  23% /
[crb@jane]: ~$ export POSIX_ME_HARDER=yes
[crb@jane]: ~$ df
Filesystem           1k-blocks      Used Available Use% Mounted on
/dev/hda3             18690576   3958640  13782496  23% /
[crb@jane]: ~$ export POSIXLY_CORRECT=yes
[crb@jane]: ~$ df
Filesystem          512-blocks      Used Available Use% Mounted on
/dev/hda3             37381152   7917280  27564992  23% /

So it's not a very good one, but it's part of the history. :)