In computing, a fatal exception error or fatal error is an error that causes a program to abort (ABEND) and may therefore return the user to the operating system. When this happens, data that the program was processing may be lost. A fatal error is usually distinguished from a fatal system error. A fatal error occurs typically in any of the following cases:An illegal instruction has been attempted Invalid data or code has been accessed An operation is not allowed in the current ring or CPU mode A program attempts to divide by zero.