The serial numbers that I used to have problems with were for software. Something would happen and the little magic file on my computer that told the program it was registered would get eaten. The program itself was fine, it just refused to run until I entered the 73 digit code that was printed on the back of the CD case I haven’t seen in the past 3 years. Fortunately now that almost all software is downloaded I just need to do a simple email search to find the string of gibberish that makes the thing do its stuff.