As recently noted by MacFixIt reader Katie Berryhill, Apple makes it difficult (but not impossible) to perform screen captures when running the DVD Player application.
"I tried to do a screen capture (Command Key-Shift-4) today and got an error beep. I had DVD Player running. When I quit DVD Player, the screen capture worked perfectly again. When I started up DVD Player again, I tried to do a screen capture before the DVD started playing and DVD Player quit by itself. Once the DVD was running again, screen capture again didn't work. Is this a known 'feature' for anti-piracy purposes? I wasn't even trying to capture the DVD screen, and the DVD playing was not a copy protected one."
Indeed this restriction is an intended feature of Apple's DVD Player, likely due to copyright issues.
Fortunately, you can workaround the protection by entering the following command in the Terminal:
- screencapture -c
This command will force Mac OS X to capture the screen as an image and save it to the clipboard.