Back in college there was a guy that gave a presentation each year about the JFK assassination, he would divide time between the various theories and it became something we went to each year.
Just last week I read another theory that a Secret Service Agent actually fired the fatal shot by accident as the motorcade sped away. He was in another vehicle holding a newly issued rifle and as he swung around to take a shot at what he thought was the assassin, his weapon discharged as he went off balance when the vehicle sped up and by a one in a million shot hit the President in the head.
Here's the article link:
http://www.clothmonkey.com/truth.htm