This is my take on it but im not a DR. i don't even play one on TV.
alcohol is processed by the liver. acetaminophen (tylenol active) is processed by the liver as well. Both chemicals can wear on and stress out your liver. If you are totally drunk, and you wake up the next morning with a hangover, you aren't supposed to take tylenol because your liver is totally stressed out from processing all the alcohol and the additional chemical load might cause your liver to fail.
I THINK this happens when you have - too much alch, too much tylenol, too much stress on the liver.