Glitches
Throwing Knives Water Trick
Get the throwing knives cheat mode and activate it. Go to the first level (The Dam). Go down to the docks at the end of the level and take out the throwing knives. Throw one into the water. It will land in the water but still be visible. Take out a gun (the Sniper rifle works best) and shoot at it. After a hit it will appear at the edge of the dock or wherever you threw them from! It even works on top of the guard towers or any place where you can reach the water with the knives and shoot them.
Over Kill
This is a cool glitch for multiplayer. Play Multiplayer in any level with any character.
When some body dies don't press start for the character to return. After they have faded, shoot the spot that the character faded in. It will make the same sound that it makes when you shoot somebody. If you shoot in the right spot the bullets will not hit the floor. After you are done pumping lead into the invisible character, press start on their controller. Find the player and they should be really bloody.
Splish Splash
You can hear the sounds of water when you shoot into toilets. You can also, however, hear the same at the complex: Go to the v walkways and fire down into he black center. You'll hear splashing like when you shot in the toilets. It's easier to hear if you use a silent pp7.
The Floating Dead
Play multi player in the Facility. (2 for best view, maybe 3 or 4) It should put a player in the air duct.(If not kill a player enough times and it will reset you there sooner or later)
Once player 1 is in the duct have that player move to the edge of the hole above the toilet but don't drop down yet. Have player 2 go to the toilet stall right under the hole and look up. Then have the player in the shaft move back and forth on the edge of the hole without falling. It should look to player 2 like player 1 is floating in and out of the hole in the ceiling. Get it to look like player 1 is floating in mid air and have player 2 kill him. He should die in mid air and fade away.