Pierre Clouthier of Progeny Genealogy has written an excellent article that I suggest should be required reading for all genealogists as well as many other computer users. The article describes how one person faithfully made frequent backups of his or her 140,000-person genealogy database but never tested those backups to see if the data was good. In fact, in this case, the data in the genealogy program's database was already corrupted on the user's hard drive but the corruption would not appear until a certain individual's record was accessed. The user kept backing up bad data over and over, probably for months and possibly even for years.
The problem is that the program would abort under some circumstances. Going to a backup copy proved worthless, as the recent backups all contained the same error. Had the problem been caught earlier when a pre-corruption backup of the database was still available, the problem would have been a minor inconvenience. However, by the time the problem was discovered, all the earlier backups had been discarded while recent backups were still available.
You can read the details in Pierre's article at http://progenygenealogy.blogspot.ca/2012/11/the-perils-of-cycling-your-backups.html. Also note Pierre's suggestion at how to prevent such problems in the future.