strange task list button

[cathayan]

Hi,

I am using Xfce4.6.1 on Debian sid, and these days I noticed this
strange tasklist button. see,
http://cathayan.org/xfce4.6-tasklist-button.png

It happens just after the xfce desktop showed up, with only one
xfce4-terminal window opened automatically, and it will turn to be
normal when I open another application, such as Firefox. After that,
all task list button will always behave fine until next start up. What
is the problem? My debian sid is up to date.

-- 
cathayan at gmail.com