My bad, i missed a part of the above post

On a Heroic Kel'thuzad kill we had last night I was hit with a frostbolt for ~28k while Mirror Image was active.

Normally the frostbolt is ~8k damage, but with mirror Image up it feels like it places a 4x damage on me. This is the second time it has happened to me on Kel'thuzad. Has anyone else noticed anything like this before or is it just me?

I casted Mirror image at 05:28'15

05:28'25.000 Kludge is afflicted by Frostbolt.
05:28'25.016 Kel’Thuzad Frostbolt hits Kludge for 28242 Frost. (3047 Resisted)
05:28'25.360 Kludge died.
05:28'26.516 Mirror Image #2 died.
05:28'26.516 Mirror Image #3 died.

Full log is here: Wow Web Stats

The way I understand it, Kel'thuzad has two frostbolts. One hits the tank for roughly 25k damage and is the one you want to interrupt. He also has a second one he just randomly shoots out at the raid that hits for far less:

Frostbolt - Spell - World of Warcraft
Frostbolt - Spell - World of Warcraft

It's difficult to tell from the log, but it looks like perhaps mirror image wore off, you had pulled aggro (or the tank was dead or something) and he splatted you with the top-aggro frostbolt.

On heroic, his single-target Frostbolt hits his primary aggro target for 30k, his multi-target Frostbolt hits everyone for 8k, and his Frost Blast hits a random target (and nearby friends) for 104% of the targets maximum health.

You pulled aggro, or the tank died, or the tank didn't pick up KT after a mind control (aggro wipe), etc, and were hit by his primary-aggro-target nuke.

EDIT: Upon browsing your log, there was a mind control shortly before you were gibbed (I see your raid members damaging each other with various abilities). Kel'Thuzad resets his entire threat table when he mind controls, and you out-TPSed your tank right at that moment, and his first frostbolt target was you.

I have experienced in some bosses (like DTK frostmage boss, dont remember the name) that dont target mirror images in his random target frostbolts, he only targets the mage.

Based on that, and what Kludge shared, could he just have passed tanks agro with his mirror images (133% threat thing), and the boss casted the 28k frostbolt (usually directed to the top threat raider) on him??

and another thing: what happens to the threat if the mage casts invisibility while the MIs are up? or while the 30s is over?

Your threat is wiped completely when you cast Invisibility during MI. If I am afraid of pulling threat when the aura fades, I make sure to cast invisibility before my CD on MI is under 2:30.

(Having the omen change helps a bunch when judging whether to invis or get a salv)

Update: I have just released Omen v3.0.6 which contains the MI/Fade changes.
Convenient download links:
Omen Threat Meter : WoWInterface Downloads : Combat Mods
Omen Threat Meter - Addons - Curse

Latest patch notes:
WoW -> Test Realm Patch Notes

Mirror Image: The Mirror Image Polymorph ability now has a range of 8 yards. In addition, Mirror Images will no longer have excessive threat values assigned to them when they are created.

Anyone have a PTR character copied to see the new MI threat values?

Old post, but: it's likely meant to be an arbitrarily large negative number and causing an integer overflow. Searching for "410065408 integer" brings it up a few places where people add 500,000,000 repeatedly, for example.

It seems to me since 3.08 that mirror image no longer effectively takes me off the threat table. Anybody else notice this?

hmmm. That is possible but I highly doubt it. If you see your threat in grey in omen its because the new update was hacked in to display what your threat would be at if you did not cast mirror image. It still reduces your threat.