I was on a MacBook on the fly, so I didn't have any external monitoring. The canvas was set to fit window, which was at about 40% of the actual frame size.
Let me give you a more specific example of the problem.
I got an image, scaled it to 50% of its size and placed it on the top left corner of the frame. Got another one, did the same and placed it on the bottom right corner. Played it in preview, everything was fine. Rendered, and both images weren't at the far left and far right corners, they where near the action safe guides. Then I threw away the renders and when they go back do preview, they're on the far corners again.