Re-routing the parade route to go down Elm Street.
Ordering the 112th Military Intelligence Group to "stand down," rather than do its job: which was to make certain that windows in buildings were sealed shut and roofs were clear of possible snipers.
Removing the two Secret Service Agents from their position on the rear bumper of the Presidential limousine (much to their surprise and consternation) at the last minute, where they might interfere with the aim of a sniper behind the President, or jump forward after the first shot, and protect the President and First Lady from any remaining shots by shielding JFK and Jackie with their own bodies. Either way, President John F. Kennedy probably leaves Dallas alive. A film of the Secret Service Agents being ordered off the Presidential limousine's back bumper, and the agent's reaction can be seen at the following site:
People will argue that this is all merely a coincidence, but I agree with Robert A. Heinlein's general assessment regarding coincidence which states that, "Coincidence is a question begging term." In other words, people say something is a coincidence when they are unwilling to investigate the circumstances of an incident with the due diligence it deserves: either because they are lazy, fearful or because it does not fit their preconceived notion of what has occurred.
Even if Lee Harvey Oswald was the "lone gunman," which I personally doubt, he was not the lone person involved in the conspiracy to kill JFK.
First of all, Oswald has the stink of CIA/ONI (Office of Naval Intelligence) all over him, from the time he was a kid in New Orleans.
Second, his "defection" to the Soviet Union has the ham-handed paw prints of the CIA all over it, especially in his return to the United States, and the lack of prosecution when he did return.
Third, his actions after his return to the United States, look mostly like a CIA operative who failed in his mission, and is trying to make himself useful to his government by trying to infiltrate pro-Communist pro-Soviet, or pro-Cuban, organizations in the United States--such as the Fair Play for Cuba Committee--as a CIA/FBI informant.
I believe Oswald being set up as a "patsy" for JFK's murder as his final "reward" for the abject failure of his mission as a "sleeper agent" to the Soviet Union. He was in the position to take the blame for the crime whether he actually shot the President or not. He simply had to die before any "outsider," such as defense counsel, had a chance to speak to him, which certainly would have occurred on Monday morning.
This leads us to poor Jack Ruby.
Jack Ruby is possibly the largest single enigma in the assassination of President Kennedy. A Jewish gangster from Chicago, Jack Rubinstein, aka Jack Ruby, operated at organized crime's periphery in Dallas during the 1950's and early 60's. He claimed to have run numbers for Al Capone when he was a boy.
Ruby initially claimed that he shot Oswald so Jackie Kennedy would not have to return to Dallas for a trial. This claim is, on the face of it, so preposterous that it lacks any credibility. He later told Chief Justice Earl Warren in an interview, that the only way that he could tell the Warren Commission the truth about President Kennedy's murder was if the Commission took him to Washington to testify, and placing him first in protective custody. This would seem to automatically disqualify his initially stated reason for shooting Oswald, because if Ruby could only tell the truth to the Warren Commission in Washington, D.C., under protective custody, by logical extension anything he said to authorities in Dallas most likely would be a lie.
I have often wondered what it was that Jack Ruby did to get himself exiled from Chicago to what was then considered the "cow town" of Dallas? I also wonder what the Mob had on Jack Ruby that allowed them to force Jack Ruby to take out Oswald in such a public manner, with no opportunity to escape?
The "rush to judgment" naming Lee Harvey Oswald as the lone gunman, without any real investigation by the Dallas Police, the FBI, the Secret Service, or the Warren Commission into the possibility of a conspiracy involving any one of a myriad of possible suspects both inside and outside of the American Military-Industrial Complex, was one of the greatest disservices ever performed by law enforcement in this country. I would call it a lynch mob mentality, but in this case, the mob was being led by law enforcement and the judiciary, while those who stood the most to gain from JFK's death, cheered from the sidelines.