Single "Magic" Bullet Theory:

The single bullet theory is the official government ruling behind the assassination of President Kennedy.  This theory was created by the Warren Commission.  The Warren Commission was established by Johnson in order to investigate the assassination.  The single bullet theory says that one bullet hit Kennedy in the back, exited his neck and then continued on to hit Connally in the back, wrist and leg. The bullet eventually falls out of Connally's leg and onto his stretcher at Parkland. The found "magic bullet" is in near perfect condition.
The single bullet theory proposes three shots: one misses limousine completely, one hits Kennedy and Connally and the last delivers the fatal head blow to President Kennedy.   Gerald Ford, a member of the Warren Commission, altered their findings by changing the bullet's point of entry from the back of Kennedy's neck, as initially written in the report, to his uppermost back.   Ford claims this change was made to clarify not to alter history.  However, the point of entry is a key component to the commission's findings that Oswald acted alone.

Source:  http://www.chron.com/content/chronicle/special/jfk/