Now, assuming that the error had something to do with the open Word document, you have to wonder why the error message didn't say something like "You cannot close this spreadsheet while the embedded document is open". Obviously at some point in the development, someone decided that "Reference is not valid" was going to be an appropriate message to show Bob Smith the accountant tracking inventory. Even as a developer, I have no idea what that could mean. But, since the only choice is "OK", I guessed that I should click it. After that, nothing seemed to happen, so I decided to try quiting Excel entirely. And then...
Ummm..... huh? What do you mean you can't quit Excel? Am I not in control of my own computer. I said quit and and you should quit. Again, somewhere in the development process someone decided that it would be ok to be in a state in which the application cannot stop. Like a bus with no brakes, all we can do is honk the horn and tell people we can't stop.
I assumed that the problem has to do with this Word document. So I just hit "OK" (By the way... things DO NOT feel ok at this point). I close Word and then try to quit word again and get...
... hmmm... So now things are not looking good. So I do what everyone would do and open up Task Manager, click on the Process tab, find excel and click End Process. So, if I could "quit" Excel, why did Excel say it couldn't?
This is a case where these messages were probably not expected to occur, but even if there is an unlikely chance of it, a small amount of time could have been spent to at least give some idea of why the program was in this state. A few minutes more, and the message could have explained how to recover, albeit a round about way. Almost anything would have been better than these messages.
Before all the Mac followers start proclaiming that this is why Microsoft is evil, let me just remind you of this:
Watch out... its gonna blow!
In Apple's defense though, most of the time they at least did a better job of telling you how to get past your problem.
However, the bomb icon is a bit disturbing and there is really no point in saying "unimplemented trap".
The bottom line is that if you are going to say something to the user about an error condition, make sure it is in a language they can understand. "Referenence Error" and "Unimplemented Trap" might as well be in Greek to most people.