Many icons using icon names that don't exist anymore or that not follow the freedesktop icon naming spec http://standards.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html

have a look at the Oxygen icon set for a good example how to name non existing icons http://websvn.kde.org/trunk/kdesupport/oxygen-icons/48x48/actions/

A few examples:

Midori:

stock_add-bookmark >> bookmark-add/bookmark-new/list-add
stock_new-tab >> tab-new
stock_new-window >> window-new
gnome-stock-trash >> user-trash					
package >> package-x-generic

Appfinder:

gtk-go-down >> go-down
gtk-go-up >> go-up

Thunar:

gtk-refresh >> view-refresh
background >> preferences-desktop-wallpaper
gtk-home(sidebar) >> user-home
inode-directory >> folder

Thunar-volman

media-cdrom >> media-optical

Xfce-taskmanager

gtk-execute >> system-run
xfce4-appfinder,  funny because appfinder uses gtk-find which is another obsolute icon >> edit-find
...
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Other icons are doubled eg. the power-manager + the battery-pugin share the same icons with different names 

the battery-plugin and many other apps/plugins failed to catorize their icons correctly(eg: xfce4-battery-full-charging >>status, xfce4-fsguard-plugin-urgent>>status)