Looks like some DVDs are starting to have problems:
http://www.smh.com.au/articles/2003/01/31/1043804519345.html
http://www.manifest-tech.com/media_dvd/dvd_compatibility.htm
http://www.smh.com.au/articles/2003/01/31/1043804519345.html
http://www.manifest-tech.com/media_dvd/dvd_compatibility.htm