The process of files getting corrupted as a result of some hardware or software failure is called data corruption and this is one of the main problems that hosting companies face since the larger a hard drive is and the more information is stored on it, the much more likely it is for data to be corrupted. There're various fail-safes, but often the info is corrupted silently, so neither the particular file system, nor the admins detect a thing. Consequently, a bad file will be treated as a regular one and if the HDD is a part of a RAID, that file will be duplicated on all other drives. In theory, this is for redundancy, but in practice the damage will be worse. Once a given file gets corrupted, it will be partly or completely unreadable, so a text file will no longer be readable, an image file will present a random mix of colors in case it opens at all and an archive will be impossible to unpack, and you risk losing your site content. Although the most popular server file systems include various checks, they frequently fail to detect a problem early enough or require a long time period to be able to check all files and the web server will not be functional in the meantime.