When the operating system or computer processor receives an instruction from a program that it does not recognize and cannot process, it may issue a command known as an Illegal Operation. Basically, it's stating the computer program is making an illegal request.
While an Illegal Operation can occur due to an instruction coming from something other than a computer program, a program issuing the illegal request is the most common cause. When this occurs, the program may close down, and any work that was unsaved is lost. If the program does not close down as a result of the illegal operation, the user can continue working, but it may be best to save any work, close the program, then re-open it to clear up any lingering errors and to reset the program.
Also see: Error