Version:           4.5 (using KDE 4.5.0) 
OS:                Linux

Applications that place an icon in the system tray that doesn't use ARGB visuals (there is a non-transparent box around the icon) spawn a contentless window with no label--the taskbar entry has an X icon, which I presume has something to do with the X-server.  When the application is closed, the X-window closes automatically.  If the empty X window is closed manually, the desktop crashes to a black screen, although other running apps still function and can be displayed.

Reproducible: Always

Steps to Reproduce:
Start any application that uses non-ARGB taskbar icons.  This includes vlc (1.0.6 on my system) amule or trucrypt among others.  You can also start other applications that place an icon in the system tray with the following prefix:

env XLIB_SKIP_ARGB_VISUALS=1

Note: not all applications will put a non-ARGB icon in the taskbar when started this way, but many will, including skype, smb4k and audacious.

In each case, you should get the empty X-window starting up along with the application.

Actual Results:  
An empty X-window pops up.

Expected Results:  
No X-window popping up.

** My System **
Motherboard: MSI K9N SLI Platinum (nForce 570 SLI chipset)
CPU: AMD Athlon(tm) 64 Processor 4000+ (2.6 GHz)
RAM: 2GB DDR2
Video: Dell NVIDIA GeForce 7800 GTX w/ 256 MB RAM (PCI Express)
OS: Kubuntu 10.04 i386 w/ KDE SC 4.5.0 from Kubuntu PPA
Linux Kernel: 2.6.32-24-lowlatency
NVIDIA driver: 256.44
Screen Resolution: 1280 x 960
X.org: 7.5
Qt: 4.7.0