At work I have a Dell 2001FP monitor hooked up to my MacBook. I usually keep my email clients and TweetDeck up on it. I've noticed that when I drag a window over an email client window, the text in the email client is still visible, almost as if the front window was not totally opaque. Obviously this is not the case, since it only happens on this particular external monitor and never on the main laptop display. Is there a name for this effect and or anything I can do to avoid it? This does NOT show up in a screenshot! It must be a physical problem with the monitor.
It's a phenomenon called Image Persistence. It used to happen to me on my original G5 iMac 20" after several years of use.
You might try to "exercise" the LCD within the projector by displaying a sequence of pure red, green, blue, and white (hex #F00, #0F0, #00F, #FFF) full-screen on your computer, but there's not really anything you can do to really fix it except get another display. Good luck!
Is it the whole window or the titlebar?
If it is only when dragging and not when it is open/selected, it could just be one of the Aero effects that you can configure under
As for not seeing it on the laptop monitor - it could simply be that it has always done it but the laptop's panel is bad quality or can't display all the colours and you simply have never noticed it. I am sometimes amazed at the difference when I drag windows between screens.