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 which web hosting companies face since the larger a hard disk is and the more information is stored on it, the more likely it is for data to become corrupted. You'll find various fail-safes, still often the information becomes damaged silently, so neither the file system, nor the admins see anything. Because of this, a bad file will be handled as a good one and if the HDD is a part of a RAID, that particular file will be duplicated on all other drives. In theory, this is done for redundancy, but in reality the damage will get even worse. When a file gets damaged, it will be partly or fully unreadable, therefore a text file will not be readable, an image file will display a random combination of colors if it opens at all and an archive shall be impossible to unpack, and you risk losing your content. Although the most commonly used server file systems feature various checks, they quite often fail to detect a problem early enough or require a vast amount of time to check all the files and the hosting server will not be operational in the meantime.