 In Part 1, Chapter 2 ("One Way Or The Other" Playstation 2 level):  Right at the beginning, there is a hole in the bricks in the wall.  Cheat yourself a grenade and throw it in. Your mission objectives  now say "I had declared a war against rats" (the Playstation 2  version has no Mission Objectives screen, but if you chuck a few  grenades in the hole the trick will still work). A bit later, after  walking through some water and taking out a few enemies on a train  platform, walk up the wide stairs. In the room at the top are a  bunch of rats with Desert Eagles fused to their bodies! In the PC  version, you can apparently do the following: Shoot one of the  ceiling lamps, hit the base of the lamp and it will fall. The rats  will now battle each other. If "your" side wins, the remaining rats  will take up defensive positions around you. If your side loses, the  remaining rats will come after you.