On the mission "Infinity", at the part where you manually fire the MAC Cannon, if you were to stand directly to the left of the cannon, a message saying "Press X to enter MAC Cannon" will pop up for a few seconds. If you press X, though, nothing happens. My guess is 343 originally intended for the player to control the cannon like in Reach.
[Edited on 12.23.2012 8:03 PM PST]