Twice my ROTF Deluxe Sideswipe has done a complete 180 on my desk, but I attribute it to the fact that he has wheels for feet and my desk is slightly tilted. How level are your shelves?

 

As cool as this could be, I don't think you can start screaming "It's a ghost!!!" until you have eliminated all possible results.

For example, for a while, my big stereo in my living room seemed to keep turning on by itself. First time I turned it off and brushed the experience off. But it kept happening! It started to get kind of creepy, especially giving my curiousity in this sort of thing. However, before I started calling the paranormal investigators, I decided to monitor it, see if there was a pattern. There was, it kept turning on at the exact same time everyday. I looked at it and, sure enough, there was an alarm function I didn't even know about that got flipped on.