Artie has already mentioned CDs going bad and the fact that brand can make a difference, and there are several other things that can happen with burned CDs. Basically they are not as reliable as many of us would like to believe. While your experience is not particularly common it is not rare for many reasons.
Commercial CDs and DVDs are not burned they are mechanically stamped creating microscopic pits on the drive surface that are read by the "read" lasers in the drive. Burned CDs/DVDs simulate the microscopic pits using a dye layer on the disc whose color is changed by the "burn" lasers and about 80% of the time the "read" lasers on a different drive will read the dye color changes the same as if they were the pits of a stamped disk. How effective this is depends on...
- The strength and precise frequency of the "burn" lasers
- The chemical makeup of dyes in the specific media
- The reflectivity of the aluminum reflecting layer of the medai. (Air infiltration will over time oxidize the aluminum reflective layer making it less reflecive layer and either making the disc unreadable or causing read errors)
- the precise frequency of the read lasers
- Different brands and even different product lines within the same brand may use different dyes and different reflective media which can effect the read and/or write performance in different drives.
- dust on the lasers and/or the mirrors and/or the sensors in a drive can effect the read/write performance of a given drive.
- NOTE: in each superdrive there are different read and write lasers for CD-R, CD-RW, D-RAM, DVD-R, DVD+R, DVD-RW, DVD+RW, DVD-R DL, DVD+R DL
- any one of these lasers can fail while the rest remain good which next to media compatibility issues is probably the most common type of failure and can be difficult to diagnose.
After reading all that it sometimes seems a wonder that optical media drives work as well and as often as they do. At one time, I would have said Superdrive failures were the most common hardware failure on Apple computers, but Apple solved that with their decision to not include Superdrives on new Macs, so now the most common hardware failure is more likely hard disk drives.
