When a program on your computer has an error, you may see a message pop up on the screen saying, "Illegal Operation." This is a rather tactless way of saying something went wrong with the program that was running. It could also be a fault with the operating system itself. The problem with the phrase "Illegal Operation" is that it seems to put the blame on you, the user. The fact is, the error was most likely caused by a bug in the program, and is certainly not your fault.
Common errors that produce illegal operation messages are divide by zero errors (no number is divisible by zero), and memory leaks where the program tries to address memory in another program's memory space. If these errors happen while a program is running, the execution comes to an abrupt halt and the program usually quits. Illegal operations can happen on both Windows and Macintosh computers, though the Mac OS X operating system is better at avoiding system-level errors.