When one sets a background picture for xfce4-panel using the panel configuration gui (NOT panel.rc), all 'button' elements borderd overlaps with panel borders, and this is ugly. Seems that panel padding is unset/zero or ignored. This is an issue because bg_pixmap[NORMAL] in panel.rc doesn't seems to work with alpha channel in png files, even when using a murrine theme with rgba active. Tested with: xfce4-panel 4.8.5 by archlinux packages. Reference and screenshots here: http://forum.xfce.org/viewtopic.php?id=6084
Hey man, the bug is still alive and kicking :D
This has all changed quite a bit with Gtk3 - can't reproduce.