The little arrow-widget on the notes plugin can change its position depending on the orientation of the panel.  This works fine when changing the panel orientation after starting up xfce.  When the notes plugin is in a vertically-oriented panel, though, and the user has logged out and logged back in, the arrow-widget remains on the right-hand side, rather than underneath.

The fix could be, to have the notes plugin detect the panel orientation at startup and position the arrow-widget correctly.