Sometimes runtime errors occur not because you made a programming mistake, but because your program encountered a situation that you could not foresee when you wrote it. This is particularly relevant when you deal with files: for instance, when you access a file on a USB-stick, and the user pulls out the USB-stick during the file processing, obviously an error occurs that you cannot really account for in your code. Every runtime error raises a so-called “exception,” and you can “capture” such exceptions if you want to deal with them in your program, rather than make the program end abruptly.