Just after you get the friction dampeners, There's is a part where you must pull a lever to drop a bridge so you can run across.  There is a grenade pack (I think its grenade) in the water under the bridge.  To my knowledge, once you pull the lever, its impossible to get underneath the bridge to get the pack.

There's a grenade door next to the bridge that allows you to get under it. The only missable item in the entire game is the inertial element.

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There's a grenade door next to the bridge that allows you to get under it. The only missable item in the entire game is the inertial element.

"

 the inertial element fucked me over - I have to start over again just for that one weapon. Thankfully I'm past level 20 so I know where every item is.

I'm on my 5th playthough. Frist was normal just to beat the game. Seccond was to get 100% complete. Third was the joke ending (lol). Fourth was the Insurgent achievement (if you want to do this, the best way would to be to check out the guide here on Giant Bomb). Now I am going to keep replaying the game until i get to level 50 =D

Since were on the topic of hard items,
 
I can't get this item in the big factory room. Their is a door on the ground and on the wall. The are both green but i can't seen to destroy the one on the wall and the one on the ground leads around it.....     If you know what I'm talking about please help me.
 
@HotSauceMagik: when the bridge is lowered, their is a green door at the end of it on the floor. its hard to see.

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There's a grenade door next to the bridge that allows you to get under it. The only missable item in the entire game is the inertial element.

"

"What's the inertial element? "

" @Milkman said:

"What's the inertial element? "

The last gun (shotgun). "

The only missable one I believe is Inertial Element but the game fucks you over so bad right before the final boss by literally removing the only efficient way navigating the complex away from you. I'm trying to go back for 100% but its so frustrating having to backtrack constantly through the lower levels. If it were not for this the game would be perfect. I've yet to experience any bugs or glitches with 6-7 hours of playtime, and one instance of slowdown. And to be honest I don't see why people cry about the aiming, it never takes more than a shot or two before it auto-aims to background or foreground its not like your waiting a minute under heavy fire to aim at bad guy. But the damn backtracking near the end is ridiculous. Seriously chair it was necessary to place the final fight in a massive area that is also the only easy way of getting around because there is no teleporters like any game in the genre in the last 15 years? 

I also found on a recent playthrough that if you get a missile pack before you're supposed to, you'll be unable to pick up the one to the left of the downdraft fan (where you're supposed to first get missiles).  I made the mistake of grabbing the missile pack from the hole in the roof of the missile turret room using an infinite foam ladder, but then found much later that I couldn't pick up that initial missile pack later in the game.

What about the item above the conveyor belts, through the furnace room? Bottom right corner of the map- I can't get that.