Dr. Jekyll and Mr. Hyde, the Green Goblin comics and The Invisible Man. In all of these stories, the situation went horribly wrong. In reality, self-experimentation tends to be less dangerous than all of that and has actually led to some great discoveries. That is not to say that it is without its dangers, but it is far less bizarre than our imaginations would tell us.
Something about self-experimentation that you probably don't think about when you hear the phrase is that most of us have done it. The whole concept is wondering if something works and then testing it on yourself. Something as simple as switching shampoos regularly to see if your hair feels better when you don't use the same product all the time is a self-experiment. Of course, these are scientific experiments in the loosest sense of the term, but it is the same concept. That's really not all that bizarre.
As for actual scientists, whether mad or not, there have been relatively few instances where self-experimentation has gone horribly awry. Carl Scheele -- a chemist -- died from ingesting his concoctions. The cause of death is presumed to be mercury poisoning. Another self-experimenter won the Nobel Prize in Physiology or Medicine. None has gotten superhuman powers from their experiments, which is a bummer. Many self-experiments don't even involve ingesting things -- though some have discovered psychedelic drugs that way. Some psychologists purposely mess with themselves just to see what will happen. Thankfully, that helps the rest of us crazies when we need help. Therefore, let's take self-experimentation off our list of bizarre things and leave room for all the truly bizarre things out there, like poisonous grenade throwing trees.