The magic mouse style back sliding (one fingered swipe right) shows a clipped view of the website if the inspector was open in that tab and the browser wasn't resized at least once prior to closing the inspector. Steps to reproduce: open a new tab open a website bring up the inspector close the inspector do the magic mouse style on fingered swipe to go back
This is a Safari issue and not WebKit. This is tracked internally by <rdar://problem/9349276>.