Solved by installing OS X 10.9 (Mavericks)
Scrolling around the hotbar was performing oddly.
When I scrolled with my mouse wheel or two fingers with my touchpad, I expected just the selected hotslot (on hotbar) would change.
When I scrolled with my mouse wheel or two fingers with my touchpad, not only did the hotslot change but also my view "scrolled" up and down or left and right if I scrolled left and right on my trackpad.
Steps to Reproduce:
1. Open a minecraft world
2. use a trackpad or mouse wheel to change the selected hotslot