I’ve always noticed more production gaffs in ANH than the other films. I can’t really say when I noticed most of them, but one I know exactly when I noticed and why. When I sat down to watch Empire of Dreams in 2004, I saw that they used some alternate takes of R2 leaving the line in the droid sale. There is a partially costumed Jawa that helps R2 get started. In the final cut in the film (every version from 1977 though the 2011 blue-ray) you can see the long human arms coming out of the Jawa costume just behind R2 as he starts moving.
When the Imperial officer comes in to tell Vader and Tarkin that Dantooine is not the Rebel base, Vader’s dialog toward the end of the scene doesn’t line up to his hand gesture.
R2’s retraining bolt is put on in the correct place but then when he is sucked into the sandcrawler it is in the wrong position.
After R5-D4 blows his top, he is back in line next to R2.
I didn’t noticed the repeat for the Tusken Raider attacking Luke until Empire of Dreams and most of the time I still can’t unless I concentrate on it.
A lot of the dialog in the Blockage Runner if out of sync.
I think I first noticed this one thanks to comments on here. When they are headed to docking bay 94, as the camera pans as they walk around the corner, one R2 vanishes and another appears, skipping quite a distance.