Could this be due to the way that you are using the mouse - the Mighty Mouse is actually only a one button mouse, with sensors to indicate where your fingers are when the button is clicked. If your left finger clicking button is still touching the mouse when you right click, it will register as a left click. You actually have to physically lift the left click finger from the mouse to ensure a right click. This could be difficult to ensure, particularly in the heat of a moment in the middle of a Halo game.
Some people, like me, find they do this automatically when they right click, and as a result, I really like the Mighty Mouse. Others don't. The general consensus is that whether you like the design of the mouse or not, it is not the best for gaming, for the reasons described, but also because it can't recognise a simultaneous right and left click.