Strange but true:
At least two dozen - four dozen, of the witnesses to the assassination thought at least one gunshot came from in front of the presidential motorcade, a claim rejected by the Warren Commission and most U.S. news organizations...
Review the eyewitness accounts, especially those of people with crime scene training.
http://jfkfacts.org/21-jfk-cops-who-heard-a-grassy-knoll-shot/#more-6381
Eyewitness testimony is notoriously unreliable.
“The next important tabulation of earwitness testimony came from the House Select Committee on Assassinations. They found only 20 witnesses who actually believed they heard the shots from the vicinity of the Grassy Knoll, and 46 who thought the shots came from the direction of the Depository. See House Select Committee, Volume 2, p. 122.”
http://mcadams.posc.mu.edu/shots.htm
Do you know that there were eyewitnesses at Pearl Harbor who swore they saw German aircraft among the attacking planes?