Poison
There have been a ton of talk of if anyone has actually been poisoned from Halloween candy given by other people. There was a specific case where kids where poisoned from eating Pixy Stix. Cyanide was inside of them; that was planted by their father for financial reasons – not an outside source. However, poison is on this list for obvious reasons as it such an easy thing to put into candy without people noticing it; poison is also extremely dangerous as well