I bought a used music CD recently and held it up to the light to check for scratches. While doing that, I noticed small holes where light shone through. So I checked a lot of my music CDs and was disturbed to see that maybe around 30% of them had this problem. What you have to do is hold the disc up directly against a lightbulb, with the data side facing you, and then you might see one (sometimes more than one) small bright hole in the disc. It helps if the top of the disc is plain and silver, it’s tougher to see the holes if the disc has a lot of silkscreening on it. Some of my CDs that have these holes are over 10 years old, some are a few years old, and even a brand new one that I opened up recently has a small hole.
Are these holes normal? You would think that these holes would cause skipping and/or other sound problems, but I’ve listened to most of that one used CD (which actually has several holes in it) and I have not noticed any problems.