Continuity mistake: While James is arguing with Tuco in the desert about the twins the sun changes in several shots.
Continuity mistake: In the desert when Tuco and Nacho are interrogating Saul, the sun and sky change half a dozen times. When he is being cut free, the sun is to his left. The next shot it's overhead. The continuity gradually improves as the scene goes on.
Continuity mistake: Out in the desert when Jimmy is tied up, they decide to let him loose. Tuco's thug goes behind him, and James has his hands duct taped behind is back. The thug bends down to cut the tap, and then the camera cuts to a rear view. As the thug pulls the tape away, Jimmy's hands are suddenly instantly in front of him at his sides and he's leaning forward.
Answer: It might be the Panavision cap he wore when directing his terrible commercials.