Bleed through

I'm seeing bleed through on windows. I have had an IM window and My Open Place opened for most of the day. I first noticed after some time that the URL bar at the bottom of the window did not stay at the bottom of the window if I maximized it. I think this happens readily, but later in the afternoon the borders of the windows were not being repainted properly. To try and explain in more detail, picture one window on top of another. The window on top should have its border completely visibile, when I make the window underneath the current window, I see leftovers from the other window where the borders should have been displayed.

Unfortunately this didn't happen right away. As I mentioned, I've had this active most of the day, i.e since about 7:30 am and noticed this behavior at around 4:00 pm. This is probably difficult to reproduce but for what it's worth I've only had an MSN Instant Messenger (through Meebo) and the www.msn.com home page for news open during that time. I haven't done anything else.