An anonymous colleague believes her Mac is taking liberties with the way it displays video. She writes:
Ever since upgrading to Leopard I’ve been getting strange lines and redraw problems. Specifically, I occasionally see a thin black horizontal line that doesn’t run across the entire length of the screen. It doesn’t appear to be program-based since it happens in the Finder, Word, Mail, you name it. And it wasn’t happening in Tiger, so it seems unlikely that it’s a hardware problem (or at least not hardware alone).
I believe you’re closer to the mark with “or at least not hardware alone.” I suggest this because my Mac Pro running Tiger has exhibited this same issue on rare occasions. Having traipsed around the Web to research the problem, the issue appears to be related to a particular graphics card and heat.
Specifically, if your Mac holds a revision 1 ATI Radeon X1900 XT graphics card, you could see these horizontal lines. They seem to appear more often when your graphics card is running hot. It tends to do so more often under Leopard than Tiger because Leopard taxes the card to a greater extent, thanks to its many swoopy graphics effects.
If you can extract your graphics card — you have a Mac Pro, for example — switch off your Mac, ground yourself, and do so. Check its cooling fan — in all likelihood it’s caked with dust. Use a can of compressed air to blow away that gunk. While you have the Mac open, take a gander inside and see if there are other clumps of dust that might be blocking its fans. If so, clean them out.
If that doesn’t solve your problem (or you have a model such as an iMac that doesn't let you access its innards), contact Apple. There are reports of Apple swapping out rev 1 cards, which display this problem, for revision 2 cards that seem to work without incident.