Revealing mistake: Just before entering Robertson's office after climbing the stairs, Spooner is behind Sonny with a flashlight. The light shines on Sonny's back and the door, but Sonny does not cast a shadow.
Revealing mistake: In the climax of the movie when Spooner and Calvin are fighting off the robots, there is a shot of Spooner standing on one of the catwalks shooting the robots as the camera spins around vertically. At the end of this shot if you look closely you can see Spooner's neck stretch out as if he was getting choked well before the robot actually grabs him.
Answer: Sonny killed Lanning. Lanning had designed Sonny with the ability to opt out of the Three Laws when he deemed it absolutely necessary. Lanning forced Sonny to promise to do Lanning a favor, then told him the favor was to throw Lanning out the window. Forced to keep his word by Lanning's own programming, Sonny overrode the Three Laws and tossed Lanning out the window.
Phoenix