Basically, when a CD is pressed, a 'stamper' is put into a machine which has the info/songs and each CD is pressed into that stamper to make an exact copy. So if a whole batch is wrong, usually its because someone has put the wrong stamper in, or has forgotten to change the last stamper over from the previous job. When you get the odd one or two that are wrong, its usually during printing. When a CD is about to be printed, they test the 'onbody' (which is the pattern/design colours) on scrap discs left over from the previous job to make sure they get the colour codes right and it matches the spec. Sometimes these scrap test discs don't get removed properly and some are left in there - as they have the correct onbody, they can be mistaken for the correct CD. Hope that makes sense and helps a lot.