When XFCE-menu plugin added to xfce-panel it dump into console next statement so many times: ***attempt to put segment in horiz list twice
This is a message by librsvg which is used to load svg icons. It's not an error and certainly not a bug in Xfce.
Well, how to disable this messages? It clutter up console.
I don't know that you can, sorry. You'd have to ask the librsvg developers.