Every user has had trouble with a scratched CD. In the case of data CDs (CD-ROM), the drive cannot properly read the CD, giving rise to reading errors. In the case of audio CDs, the CD skips when we play it.
The first thing to do when coming across a CD with a read error is to clean it, to check whether it the error is not being caused by a dirty surface. You can even wash the CD gently with a little detergent, using your fingers to clean it (avoid sponges, since they can scratch the CD). If the error persists, try reading or playing the CD on another drive. If another drive (or CD player, in the case of audio CDs) gives the same result (read error or skipping, in the case of audio CDs), it will mean that the CD is scratched.