I have a 24inch 2.8GHz iMac bought Thanksgiving of 2007. I convinced my mom to buy a 20inch 2.66 iMac this month. It worked fine for 2 weeks then shutdown and would not boot at all. Took it back to the Apple store, they replaced it on the spot. So the new 20inch is also doing fine, but I noticed this mouse behavior on both 20in 2.66 machines. If you hold the mighty mouse in your hand with both fingers over the effective button1 and button3 spots and you move in the slightest the mouse sends a button4. What is really odd, the mouse from my 2.8 24in will do it too, but only if its plugged into the 20inch. And the 20inch's mouse doesn't do it if plugged into the 24inch. It did this right out of the box, and after all the Al Keyboard and Software updates. Is this some new behavior? It's very annoying, because I can't hold the mouse without setting off button4 now. So I've just set button4 to be off. Or is there a setting I'm missing that is causing this?