When dragging taskbar buttons to the top (don't have the bug when dragging to the bottom), the green arrows don't hide themselves when the mouse pointer leaves the taskbar button. Then, every button between the one you want to move and the position of the mouse pointer have an arrow. See screenshot. I was dragging The OneNote button "Semaine du ..." to the top.

Windows XP, custom Vista theme

I confirm that this issue is fixed. Thanks Jon!