The Program Has Performed an Illegal Operation and Will Be Shut Down
In the computer world, the word illegal is used to describe a programming operation that is not allowed. The computer programmers could have used the words prohibited or corrupt instead, but they did not.
Another popular word to use is invalid, as demonstrated by the next several error messages. The word is pronounced "in-VALid," as in "not valid." It is not "IN-valid," as in "incapacitated by illness."
Anyway! The program has done something beyond your control. Too bad. Shut down the computer. Restart Windows. Start over.
Don't bother contacting the "program vendor." Odds are that the vendor does not want to hear from you, knows about the bug, and won't fix it. You can try if you want, but my experience has shown that they really, really don't care. And, that is too bad.
Stack Overflow
Technically a programmer's error, a stack overflow happens when a program runs low on a specific type of memory. The memory is called the stack, and it is used to store information for the program - like a scratch pad.
The problem is that the stack has only so much room and a sloppy or damaged program can use up that room quickly. When that happens, the microprocessor must take over and rescue the program, lest it bring down the entire computer system. The microprocessor steps in by halting the program and issuing a "stack overflow" error message.
There is nothing you can do about it other than complain to the program developer.