Jeff,
I have this problem constantly and it is easy to reproduce. Create a System Tray (I have mine NOT showing as a drawer, "Always on top" is checked, and the "User Zoom effect" is checked). Start some app that can be exited via the system tray icon and make sure it starts before other apps that have System Trays (i.e. it will be on the left of other icons). Exit this app from its system tray icon so other icons shift left to fill in the gap. Now, mouse over any of the shifted icons. You'll notice this bug. It appears to me to be a simple array off by 1. When mousing over, the effect is created in the original spot (one array) but the icon is correctly positioned when the mouse is not over (some other array).
**User's Note: The way to fix it, temporarily, is drag the right-most icon like you're going to change its position, but put it right back to its original spot. This does a re-order on the icons and corrects the problem.
(Programmer's note...As a temporary fix if you can't find the array problem, if an icon is removed/app is closed, you could call the re-order function right afterwards)
Hope this helps...