On MacBook and Mac mini (Early 2006) computers, areas around windows or under the menu bar that should show a drop shadow may instead show pixilation or distortion. This issue can occur when system memory is depleted or completely used up.
If this occurs, update your computer to Mac OS X version 10.4.7 or higher, available via Automatic Software Update or from the Apple Support Downloads page at http://www.apple.com/support/downloads/.