In the scene where Neo first meets Morpheus, Morpheus turns to Neo and puts him arm behind his back. The scene switches to Morpheus' front, and you can clearly see that Morpheus' arm is now by his sude. It switches again to his back, where can see that the arm is again behind him.
Yes, I didn't notice this until maybe the tenth time I watched the film. It's probably because the scene was filmed at different times. Such as the shots of Morpheus were filmed one day, the shots behind him were filmed another day, and the shots of Neo were filmed on other days, or something like that.