Created attachment 2050 
picture showing the occurence of the bug

You got to see the cropped screenshot i supplied below which sums up the issue MUCH better than words.

the problem always strikes the same applications, and their icons always appear as dots, where unaffected applications have their icons rendered correctly at all times. shortly, there is no randomness involved.

to make things worse, click and right click actions are also captured on a single pixel, BUT (you didn't see this coming), its not the black pixel, its roughly a few pixels to the upper left of the black pixel.

This bug drove me real crazy when i switched my ISP, had connection problems and had to switch back and forth between two network settings. The Network Manager applet is one of the affected applications.. now picture me trying to find that right-click-pixel by trial and error over and over again.

in the screenshot, you see Network Manager and Emesene (a msn messenger client) icons, or should i say you don't see them :)

Note: the large empty space with a hint of a icon on left side is the opera icon, it's rendered incorrectly due to a widely known bug in opera's linux version, and totally unrelated to the issue i described here.

thanks for your time and hopefully this will be my first step in helping out the xfce project:)