A lot of lemons, or CDs really not designed for lots of computer use. I have found that certain CD media is of very poor quality, and CD drives of computers are especially sensitive to the CD media. While this report is quite old in many way it holds true today:
http://www.macintouch.com/cdrfailure.html
Things to watch out for on CDs:
1. Copy protection ring - forget it. These aren't true CDs.
2. Sticky labels - can easily get melted enough that pieces of the label get stuck in the drive.
3. Writing not kept clear of edges. There is a portion of the edge that actually lets light through, and if it is interfered with it could give the CD drive a headache.
I only trust media by Verbatim, Sony, Imation, and Kodak. Memorex and Maxell media I've had some very bad luck with, and generic CompUSA media as well.
I only trust media stored in jewelcases, and not media stored on spindels, since they can scratch each other on the spindels with the track side hitting the label side of the other CD when shifting in the spindel.
Software wise, if the CD isn't formatted Mac, ISO 9660, or PhotoCD, don't use it on your machine as it won't work.