Master_Shake, I love ATHF. Also, my intentions are to prank my fellow employees I would make sure they didn't execute it and lose a ton of work though.
Keith: I like your style. How did you cause the fatal error?
No error, just a faked dialog with a random error code. When it fires, it looks like the real deal, with the option to save the drawing. Once the document is saved, AutoCAD closes and the user must go back into AutoCAD.
In some blind tests, I found that after a couple of times, users didn't try it anymore. Something about saving a huge drawing, restarting AutoCAD and then re-opening the drawing made them quit trying to use it.
I guess, it isn't so much about telling people "This product isn't licensed" because they will continue to use it anyway because the vast majority of people could not care less. If you make then restart AutoCAD every time they try to use an unlicensed product, then they will a) license it b) stop using it c) something else that I don't care about.
To be fair, I never put it in any commercial software, only that which is given away under the premise that they MUST register it with me after a short while.