I actually did come pretty close to this during my month-long water fast, there was little shit nugget I couldn't poop out that was literally poisoning me for like two weeks into the starvation, and as my rectum got lazy thanks to lack of any job to do, I couldn't force myself to do a single bowel movement. The bastard moved for a cm per day best. You cannot imagine the relief I experienced when I finally managed to push it out, and the toxic monster was just the size of a fingernail. Again, I'm not exaggerating when I say it was poisoning me, all sorts of nasty toxins are released back into the body by a stale poop. After that I just didn't poop at all until the fast was over.
Good times, I need to try it again.