Arrow of time
The arrow that was first seen in Desmond's glimpse of the future killing Charlie, and later missed killing Charlie through Desmond's intervention, might be representative of the Time's Arrow. This is an idea which was first brought up by British astronomer Arthur Eddington and made popular by Stephen Hawking's "A Briefer History of Time". There has been a novel of the same name written by Martin Amis, as well as an episode of the Sci Fi shows "Sliders" and "Star Trek: TNG". The idea behind the Time's Arrow revolves around the movement of time in one direction, but allowing it as well to move in the completely other direction. Originally when Desmond saw the arrow kill Charlie, he saw that he would be reunited with his love Penny. That was Desmond's driving force behind looking for the parachutist. When Desmond stops Charlie from getting impaled by the arrow, he changed time, or changed the direction of "Time's Arrow". Considering Desmond may have affected time before, this representation may be accurate.