Some studios (Disney is one of the worst) put stupid copy protection measures on their DVDs that make them difficult to play on anything but a DVD player. That's a great way to tick off your completely legitimate paying customers.
One of the common ones is to make something other than the first title the start of the movie, or worse, to put one or more corrupt copies of the start of the movie, then have the actual movie start on title 74 or something. This makes it hard to play the DVDs on Linux with mplayer.
To work around this on a Linux machine, use vlc to find the true start of the movie. File... Open Disc... DVD (menus) and start playing the movie. (Feel free to simply watch the movie at this point). Wait until you get to the first scene. Then in the Playback... Navigation dropdown you can find the title number. Let's say it's 37. Then you should be able to play the movie with mplayer dvd://37