There has been renewed interest in finding out what non-coding DNA (aka 'junk' DNA) actually does. Some prominent theories are that some kinds of non-coding DNA are epigenetic controls, but there isn't a complete set of theories yet. There ARE transposable elements, sequences of DNA that can essentially move around in your genome at will, and cause problems by doing so: imagine getting a transport protein sequence in a stem cell cut in half. Even then, these are kind of like DNA parasites, not junk.