What can be on a cd that.....

I have some old cds that have been through the mill. Marks all over them. Scuff marks, you name it. They play without a skip, glitch, nothing. Then I can play a brand spanking new cd and after a little skip, jump,--looking at the cd: Nothing. Playing it back the second time after blowing some air on it. No problem. What types of things are on cds that can cause a skip, but not be permanant to the cd? Can, merely, a dust particle or the like, cause a sound effect? I hope I'm making this clear. thanks in advance. peace, warren