Why do some CDs have trouble reading?

Designed to squeeze the most out of a CD is the DVCD or Double VCD where a non-standard CD is overburned to include up to 100 minutes of video. This format is seen only in China and the DVCDs are playable on many DVD or VCD players though some CD-ROM drives and players have problems reading these CDs mostly because the groove spacing is outside specifications and the laser servo is unable to track it.