A favorite personal theory of mine relating to the JFK assassination (one that I made up and know is BS, but is just too funny to imagine):
Following the clusterfuck that was the Cuban Missile Crisis, Castro was pissed off that the plan to place Soviet missiles into Cuba failed, and still held a grudge against the CIA for their numerous attempts to kill him. Wanting revenge, he had his intelligence service contact the American Mafia to plan an assassination of Kennedy. The Mafia, feeling betrayed by Kennedy despite their support in getting him elected, only to be hunted down by Robert Kennedy, agrees to the Cuban conspiracy to assassinate JFK. They contract out to some ex-Nazi mercenary from South America to cover their tracks, and plan to kill Kennedy when he visits Dallas, placing their sniper at the Grassy Knoll. On the day of Kennedy's trip to Dallas, they had their sniper ready in position, ready to fire... only for lone nutcase Lee Harvey Oswald to cockblock them and kill Kennedy first.