The process of files being corrupted as a consequence of some hardware or software failure is called data corruption and this is among the main problems which hosting companies face as the larger a hard drive is and the more info is stored on it, the more likely it is for data to get corrupted. There are a couple of fail-safes, but often the info gets damaged silently, so neither the particular file system, nor the admins see anything. Because of this, a damaged 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 disk drives. In theory, this is for redundancy, but in practice the damage will get worse. The moment a file gets damaged, it will be partly or completely unreadable, which means that a text file will no longer be readable, an image file will present a random blend of colors in case it opens at all and an archive will be impossible to unpack, so you risk losing your site content. Although the most commonly used server file systems have various checks, they quite often fail to find a problem early enough or require a long period of time to check all the files and the web server will not be operational for the time being.