The entire WebKit view can shift left and right while scrolling. This happens when scrolling up and down through a panel on it, then continuing to scroll when the edge is reached. Really, it's scrolling horizontally at this point, and WebKit begins to do that once it can't scroll vertically anymore. Though I haven't tested it, this should be even more noticible when using a multidirectional scroll ball (i.e. Mighty Mouse) or the two-finger trackpad scroll, as that lets you scroll sideways at all times. Two ways to reproduce: 1) Common way (scrolling past end of panel): Open up a panel in the Web Inspector, and scroll down to the bottom using the scroll wheel on your mouse. Then keep scrolling. Then scroll to the top, and keep scrolling. (I've included a quicktime video of this. Mouse is invisible but it's there.) 2) Just scroll sideways: mouseove the web inspector and hold the shift key down and move the scroll wheel, or using a multidirectional scroll ball like the Mighty Mouse or the two-finger trackpad scroll should work too.
Created attachment 15175 [details] Horizontal scrolling behavior demonstrated
This was fixed in r23735.
*** Bug 14306 has been marked as a duplicate of this bug. ***