I use a mouse with a scroll wheel between the two buttons. I often find that I click on a dropdown list and then try to use the wheel to move the page up a bit. As I have just clicked on the dropdown, what actually happens is that the selected item changes instead.
Normally this is not too much of an issue, I just go and reselect the option I wanted and continue. It does become a problem, however, when a web developer has used the OnChange event to go back to the server and reconfigure the form. This is normally a slow process and stops my train of thought long enough to really bug me.
So please I ask everyone that needs to reconfigure based on a dropdown selection to please put a little 'Go' button next to it instead.