In our Windows apps (keep in mind that I work with others writing apps for more than Windows) we've seen that come and go as machines and apps update. It's not an unknown issue but has no definitive cause or cure.
Most of the time I find it's a video driver issue or in the case of Flash, their change to hardware acceleration which we turn off a lot.
Wish there was one cause but if there is, it's this -> The PC is a hodgepodge collection of hardware and software from some thousand makers and authors. For now it's amazing that we are getting this good a result.
Bob
Just applied the fix-it issued by Microsoft for IE8. Running XP SP3
http://forums.cnet.com/7723-6132_102-573705/microsoft-out-of-band-advance-notice-fix-it-solution/
Since the fix-it was applied, noted that another browser used, FF 15.0.1, no longer is displaying artifacts at the bottom 10-30% of the browser window when scrolling the page up or down to view content. Did not matter what site was viewed, some sites affected less or more than others.
This artifact was quite noticeable and annoying; it was a transient black area that appeared only in the bottom with no web content displayed. It seemed to have a flickering effect on the bottom part of the window when it appeared as scrolling was called. Black area always disappeared when window was not scrolled.
Anyone else note this behavior? Always use a sandbox when using a browser, whether IE or FF. What was a possible cause?
Not running any version of java on this system. Running the latest version of Flash Player.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic