BEagle,
I think that's a bit harsh. From the sound of this thread, Confabulous won't make that mistake again. So job done. I think it's a pretty good technique, let the student make the mistake but also to see what could happen. A far more effective demonstration than just telling a student what could potentially happen.
Don't worry Confab, you aren't the first and you certainly won't be the last to make a mistake. The key is that you learn from it. No-one is born a sky-god!