Probably a side effect from the lightsabers being predominantly physical effects, during the duel, you can actually see whisps of powders or particles coming off the lightsaber props. The highlighted box has a particularly dense cloud.
Twice during the lightsaber duel, Ben and Vader inexplicably change sabers as the color jumps from one sword to another. Here, Vader is swinging the blue sword while Ben holds the red one.
Here's Vader parrying with his blue sword as Ben lunges with his red.
Okay... this is just embarrassing. Look at the wire trailing from the lightsaber up Ben's sleeve.
Lightsabers appear to have the flexibility of a pool noodle in this shot.
Another embarrassing one, not so much for the failure of the effect, but for how long it lasts on screen. As Ben backs away from Vader, look at his saber. You can see right down the metal rod, and all that is lit is lightbulb tip.
Sure, these effects are primitive, but all these stills are from the Special Edition! Surely they could have cleaned this mess up. Furthermore, the Behind the Magic CD states that great emphasis was placed on fixing up the saber effects in Star Wars. I don't buy it. Short of Luke and Leia kissing in Empire, this is the most unfortunate part of the trilogy.
The lightsaber duel seems to have been shot mostly from one angle, and when Vader and Ben switch sides, in order to maintain continuity, the negative is flipped so they are standing in the right place. Such reversals can easily be spotted by looking at Vader's chestplate. The inset is its correct layout.
This is one of my favorite Star Wars "bloopers." Han and Chewbacca run into a doorway, looking at the Falcon. Then, Luke and Leia run in, supposedly dashing in at top speed. But watch as Han and Chewie run in. Note that you can see Luke and Leia’s shadows on screen right, ready to run in. The highlighted inset is the spot to watch.
Star Wars production designers kind of "cheat" when having Artoo change from two-legged to three-legged mode. In his three-legged mode, he has metal braces connecting his feet to his trunk. These braces have no corresponding version on the two-legged version. They just kind of "appear" when Artoo goes tripodal. That’s why when you see Artoo change from two to three legs, he never travels far before they cut and replace the Artoo unit. When the Rebels make a break for the Falcon, you can see just how unsettled the three-legged Artoo is without those braces.
Another shot that could have used improving. The lightsaber passing through Ben is acheived by a brown animated matte obscuring the lightsaber that cuts through Vader. Ultimately, a very poorly realized effect. It would have made much more sense to have given David Prowse an unlit lightsaber prop, and have the blade exist only in animation, that way, the animators can cheat it passing through Ben by drawing only the visible portions of the blade.
Luke opens up on the stormtroopers. The second one from the left (though he moves around, eventually) is having a hard time keeping his shoulder guard on.