Rod Speed is right !
Many markers contain alcohol, which dissolves plastic composites. That's
the reason why special CD/DVD markers are available.
Concerning the labels, there's another important potential issue. Many
CD/DVD readers become very hot, and this dissolves the glue of the label,
resulting in air bubbles between label and disc. In that case of course,
the disc becomes unreadable. I' ve encountered this problem with many
label brands, except one, which is Neato. They're more expensive, but work
fine. The best solution is to print directly on the disc (Canon or Epson
inkjet printers). However, be careful with the inks you use, as many
(replacement) inks can smear, even after a few months.