Six-Armed Spider-Man: Peter Parker at one point had enough of being Spider-Man. So, he brews up a chemical cocktail intended to terminate his spider powers. When he wakes up after drinking the mixture, he notices a ghastly thing: he has six arms! The potion increased his spider powers rather than nullifying them!
Silver Age Lizard: Curt Connors was a gifted surgeon who went into a war to help his country, but his arm was injured in a blast and had to be amputated. He later became obsessed with uncovering the secrets of reptilian regeneration, and created serum taken from the DNA of a reptile. Connors ingested it, and his arm did indeed grow back. Unfortunatley, it had one side effect: Connors was subsequently transformed into a reptilian monster!