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Hidden Costumes
Beat the game on different difficulty settings with either Chris or Jill to get the closet key. You will find it in your item box when you start a new game and you can use it to put on some sexy new costumes for Jill -- and to get some new duds for Chris at well. Each difficulty setting will get you different costumes, so be sure to play through the game multiple times to get it all.
Real Survival Mode
The hidden mode, Real Survival, is unlocked automatically when you beat the game with either Jill or Chris. Basically, it gets rid of the magical ability of the item box. So if you put an item into one item box, you can only take it out of the same one. No magical beaming. This makes the game a whole lot more difficult -- but also a lot more tedious.
Invisible Enemy Mode
Clear the game on Hard mode to unlock the Invisible Enemy Mode. In this mode, you can only hear the zombies -- they will be invisible until you make contact with them. Have fun.
Rocket Launcher
Remember the Rocket Launcher? To use this ultra-powerful weapon, you have to beat the game once with Chris or Jill in less than three hours in Normal or Hard mode. It's tough, but it can be done. Once you've completed the game (don't forget to save!), you can start a new one with the Rocket Launcher to really kick some zombie ass.
Samurai Edge
Beat the game with either Chris or Jill in less than five hours (total playtime) in Normal or Hard mode and you'll be able to use the Samurai Edge, an upgraded handgun with unlimited bullets.
Unlimited Grenade Round Trick
This is a step by step guide to getting this glitch to work. Follow these instructions or it will fail.
Requirements: You must have the grenade launcher and two types of ammo (e.g., Grenade Rounds, Incendiary Rounds)
1) Put all items in item box
2) Put Grenade Launcher in upper-left item slot
3) Put Incendiary Rounds in upper-right item slot
4) Close box
5) Open inventory
6) Equip Grenade Launcher
7) Open box
8) Select Incendiary Rounds from inventory
9) Press ''A'' twice
10) Move cursor to Incendiary Rounds inside box
11) Press ''A'' twice
The amount of Incendiary Rounds in the Grenade Launcher will now increase. The number value will be graphically glitched. Close the box and open inventory to fix that. I assume the value exceeds the possible amount, so the game automatically reduces the rounds to 240 to prevent data corruption. Enjoy.
This also works for other types of grenade ammo... But why you'd want anything other than Incendiary Rounds is beyond me.