It is not a gaffe. When the first Star wars film was shot, the actors used actual sword-like objects. They were handles with small motors and into them were set a rotating wooden rod. This rod was covered with basically Scotch-Lite Reflective Tape, wound up around it diagonally.
As the actor’s did battle, the rods rotated, catching hard light set off to one side and providing a bit of flickering. Of course, the light-saber effect is a matte of blue , red, etc. As the moment approached ( and I am well aware of this moment in the duel named above ), both actors crossed " lightsabers" and moved them down and towards the lens of the camera. For a few frames, the matte insert of the lightsaber simply didn’t track, they were inserting bars if different lengths to compensate for the perspective as the lightsabers moved around and circled in the air. For that brief moment, you do indeed see the end of the wooden dowels and just a flash of the Scotch-Lite Tape.
It seems to me that it wasn’t feasable to make an oval of " lightsaber" blue matte for those few frames, and so it wasn’t matted over and we all get to see a bit of movie prop magic. I love that shot, to be honest.
It makes me appreciate the simplicity of the prop, and the elegance with which our minds make the leap of faith, so that when an arm is severed by Obi Wan in that Cantina, we accept it as a powerful weapon.
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