Note from the dev team.

In the ''You Know The Drill'' level, tether the Predator. Then pick up a secret chip. Look at the mission objective in the pause menu for a note from one of the level designers. (The designer himself hinted at the easter egg in a GameFAQs MA: GitS board topic.) Contributed By: doninss

Play as General Corrosive in Multiplayer

First, unlock Corrosive City in multiplayer, which will require 138 chips. After that, go into the main multiplayer menu and press the Edit button (look at the bottom of your screen). Forward to the edit menu, scroll down to the 'Only Vehicles' option, and turn it to 'None'. Essentially, you are turning off the vehicles. Finally, load up the Corrosive City map and hit either jump pad in the center of the level. Take the next few jump pads up until you're on the platform with the control console. Activate the console to gain control of General Corrosive. Note that failure to turn off vehicles will result in an out of order message from the console. Contributed By: Cold NRG