That's very plausible.
http://www.leagueoflegends.com/board/showthread.php?t=49359
The post was made a year ago, and even recently there's been a question asking if it needs to be updated.
Disregard that section of my post, then.
http://www.leagueoflegends.com/board/showthread.php?t=49359
The post was made a year ago, and even recently there's been a question asking if it needs to be updated.
Disregard that section of my post, then.





Armor reduction and armor penetration are applied in this order:
1. Percent reduction (
Bushwhack / Pounce)
2. Flat reduction (
Black Cleaver, Puncturing Taunt, etc)
3. Flat penetration (
The Brutalizer,
Sunder
, Runes of Desolation)
4. Percent penetration (
Last Whisper)
1. Percent reduction (

2. Flat reduction (

3. Flat penetration (


4. Percent penetration (



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Ace Ventura wrote:
Armor reduction and armor penetration are applied in this order:
1. Percent reduction (

2. Flat reduction (

3. Flat penetration (


4. Percent penetration (

Confirming this, and adding that the first two *can* go below 0, but the last two *cannot*.
Note: It is currently impossible to get more than 100% percent reduction on a single target, but in theory, 200% reduction should result in negative armor, etc.

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I do know that armor reduction can lower their armor below zero, and the same goes for MR reduction. If you've got an Amumu at level 18 attack a blue buff Golem (which already has -30 magic resist), you'll see his MR drop to -65 as soon as you land that first attack. While you might waste potential on the armor penetration, the flat armor reduction from Stark's and Black Cleaver should still apply. Oh, also:
From the official League of Legends website:
"ARMOR PENETRATION
The order in which Armor Penetration is calculated:
Base Armor+Items.
Armor After Reductions from status effects/skills/items.
% Armor Penetration.
Flat Armor Penetration."
This is interesting. I never knew that Last Whisper's effects proc before flat armor penetration... We may have to rethink our opinions in this thread.
We'll need confirmation about this ingame, since every other mathcrafting source states the opposite, some with screenshots - and the patchnotes some while ago stated that the order got changed as well. Maybe the info is just outdated?