Use random untested programs. preferrably obscure ones from github with no forks or bug issues. For instance, don't the language's search methods, instead make a call to use the system's grep. But not the actual grep command, but a modified one - like grep-contrib. Which in turn needs to be compiled with a custom compiler, gcc-contrib.
and obviously, dont document any if this. make the next poor soul look at the code and find out.