It's abuse of the contest rules, which encouraged at the IOCCC. You just have to be absolutely certain that it really IS "technically valid"[1] or it risks disqualification.

This one abuses the idea that they specify how the contest will be judged, which this "program" satisfies, as long as you don't look at the source.

    $ make smr
    cp smr.c smr
    chmod +x smr
    $ ./smr > smr.dup
    $ diff smr smr.dup
    $ echo $?
    0

Also, it is a valid C file!

    $ stat -c "%s" smr.c
    0
    $ gcc -Wall -c smr.c
    $ echo $?
    0
    $ stat -c "%s" smr.o
    927

The judge's comments for this one explain this in a bit more detail:

http://www.ioccc.org/1994/smr.hint

[1] the best kind of "valid" :)