Many people use computers in a transient way - to send email and to write and print out letters. These people don't use the computer to store any important information, so they don't care much if a file is damaged or goes missing. So long as backups are taken, the data is usually safe. Accidental deletion by the user.
Imperfect backup restore following disk failure.
Slow disk corruption. Faulty disk utilities. Ultimately, there's no direct way of telling if your files are safe without having a look at them. However, checking even just a small number of files manually is very time consuming. Technical Outline FreeVeracity provides a way to monitor the integrity of your files.