There was a bug reported on this guy here:

http://forum.square-enix.com/ffxiv/t...-damage-shield

I think I have additional information, but when I tried to reply to the thread as instructed by the second bullet in the "Rules on In-Game Bugs" section (http://forum.square-enix.com/ffxiv/t...-THIS-FIRST%29), I get the message
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Here's the information I have for this boss fight:

-------------------------------------

Thread Title:
Magitek Colossus Rubricatus

ISP:
CenturyLink

Type of Internet Connection:
DSL

Internet Connection Speed:
40 MBPS

Date & Time:
24 November 2013, 2:20pm CST Time

Frequency:
Once

Character Name:
Doctrin Brutal, Taryn Brutal, Aniya Dahnia, Zeref Discord, Colline Licious, Madison Paige, Arka Striff, Erdrick Tantegel

Race:
Race of your character when the issue occurred.
Various

World:
Instance, Castrum Meridianum

Class/Level:
Various 50s

Area and Coordinates:
(9, 6)

Party or Solo:
Full Party

NPC Name:
None

Monster Name:
Magitek Colossus Rubricatus

In-Game Time:

Steps:
Detailed steps on how the issue occurred.
One of the mechanics of the fight is that several members of the "8th Cohort" appear in the fight. When this happens, shortly afterwards an on-screen message "Damn it, that colossus is enhanced! Hold on -- I'll try to think of something!"

When this happens, there are a couple untargetable mobs that have these little pink lines that go to the boss. I was recording video in today's instance and when I was looking over the footage on the bug, I noticed that in the bug case, the little untargetable mobs with their pink lines didn't spawn when the on-screen message occurred. After the raid intentionally wiped, we were able to kill the boss without this problem reoccurring. I hope this information is helpful in finding the cause of this bug.