Continuity mistake: At the start of the film when Del is in the USR building and is looking over Dr Lanning's body, if you notice the shoe changes position from being not far from the body to some distance, also, it changes direction from the first time we see it, we are looking at the heel of the boot or the front, then it is completely side on in the next shot. (00:10:55)
Continuity mistake: At the end, when Dr Calvin, Del, and Sonny are in Robertson's office, when Sonny starts to trick Viki, Calvin has her body side on, then she is completely facing Sonny, then back to side on, then finally back to facing Sonny again. (01:27:55)
Continuity mistake: At the beginning of the film when Del goes to USR Robotics, there is a shot that pans up to Dr Lanning's body, if you notice his eyes are either closed or barely open, yet in the following shot they are much further open. (00:10:50)
Continuity mistake: At the very beginning of the film when Del wakes up he reaches over to turn off his alarm clock. The red book, whose cover was partially under another book in a prior shot, has now moved. (00:01:55)
Continuity mistake: Close to the start of the film when Del and his Chief are in the bar, in one shot we see Del drink his bottle with two fingers on the bottle, in the very next shot he is suddenly holding it with only one finger around the neck. (00:32:45)
Continuity mistake: During the car chase the copilot seat's glass swaps between being opaque to transparent.
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