Sedation/liver issue. The tylenol (acetaminophen) component can cause liver damage if too many are taken at once. The butalbital will cause sedation that cannot fully be overcome by the caffeine.
Can be fatal. Butalbital is a barbiturate and in overdose can lead to coma, suppress the breathing, and lead to death. Apap = acetaminophen; overdose can lead to liver failure and death. It is very important never to take more of a medication than prescribed or, in the case of over-the-counter meds, never to take more than on the label unless otherwise directed by your doctor. If overdosed, seek emergency help.