Symptom
A partial screen shot (Shift-Command-4) does not successfully capture a Classic window's contents.
Solution
To successfully take a picture of a Classic environment window, take a full-screen shot by pressing Shift-Command-3. A numbered "Picture" PDF file will appear on your desktop for each screen shot. You may then use an image editor to crop the image to the desired view.
Partial screen shots (Shift-Command-4) work with native Mac OS X windows, but they will not capture all of a Classic application window's contents.