Media Errors Happen
It's true. Recordable media, like blank CDs and DVDs, aren't perfect. Sometimes, a disc is going to exhibit errors while you're trying to write data to it.
What I don't understand is why developers creating backup software don't take this common occurrence into consideration when designing their software.
I've complained about Nero Backup aborting a backup many discs into a backup set, and I just ran into this behaviour again with Picasa.
I asked it to back up my photo library, and part way through the second DVD, it gave me an error, and aborted the backup.