If you highlight text in xfce4-terminal, and then immediately paste it into the same session using a keyboard shortcut (I do this all day, and I don't use the mouse, because it's broken :p ), then frequently (one in 50 times or so) the highlighted text is not synchronised from the x selection to the clipboard.  There is a delay of up to 5 seconds it seems before this text is available.  This can be quite annoying, as what was in the clipboard is not always appropriate content, and the most frequent operation after select is paste.