JFK, I know it's like the mother of all theories...
I was reading the complete warren commission report and there's something fishy about a guy Lee Harvey Oswald worked with at the book depository... a guy named Jack Dougherty. Very odd character, in-fact all of the staff seem a bit fishy to me.
Also interestingly one thing you never actually hear about is where the motorcade was traveling too - they were going to a luncheon/speech at the "Trade Mart". It seems to get lost in the midst of all the assassination drama.
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the dumbest one for me is the Moon Landing Hoax one - the USA went to the Moon and the Soviets almost did, so shut it!