I'm fairly sure this is a bug and not intended because it's not in Twisted Advance's tooltip. But if Moakia at the perfect time he can negate damage. So far I've only seen it against Nunu's ultimate Absolute Zero.

Steps to reproduce:
During Nunu's ultimate Twisted Advance to any nearby enemy
Moakia will take no damage

This is intended behavior. Maokai becomes untargetable during Twisted Advance while he is dashing.

You can also use this to "reset" turret aggro off of you and onto another allied unit.

Is that stated in the tooltip? Because that is a pretty significant thing :X

I don't think the word "untargetable" is used... something about a "transforms into a cloud of energy". I'll speak to Volty about what he thinks the tooltip should contain. Thanks for pointing out that this wording might be insufficient to convey the what the ability does.

No problem, playing Elise and Fizz I know how significant becoming untargetable can be

If the intention is to make him 'untargetable', then why is Nunu missing his giant 'no-need-to-target' aoe spell?

I know for a fact if you time it right you can, with Brand as a great example, hit Maokai mid Twisted Advance and do damage to him with Pillar of Flame and/or Seer. In the same respect you can hit Fiora mid ultimate (name eludes me right now) and the few other champs I can't think of right this second as well.

If the intention is 'untargetable', then Maokai, so long as he is still in Nunu's Ultimate, should still take full appropriate damage. If the intention is 'un-damageable', then he should take no damage at all.

On a side-note, if he is 'un-damageable' in Twisted Advance, this would be the equivalent to a mini Kayle ultimate.

Also on a side-note, if you have Brands passive already applied to Maokai before the advance, he does his advance, you Seer him before he finishes traveling and lands, the stun application will still take effect but ONLY after he lands, and with the timer of what is left of the stun factored in after travel time. This also does not prevent the root, but does prevent any action done by Maokai for whatever duration is left of the stun. Trying to simply this here, Brand lands Pillar of flame, Maokai Advances, brand lands seer, thus stunning him, if it take 0.5 seconds before he finishes his Advance and roots his target, and the stun lasts for 0.5 seconds less then full intention even though it TECHNICALLY should last the full duration after the Advance is complete.

Just my two cents, hope it helps.

It's also possible with some champs to hit Mao when he's travelling and knock him back/up and interrupt TA entirely, doing damage as normal.

• Maokai will no longer become untargetable or dissapear after casting Twisted Advance

That was a patch note in 1.0.0.144 (Diana) 2. hotfix patch (August the 7th).
Also called the Zyra nerf patch.

http://leagueoflegends.wikia.com/wiki/V1.0.0.144

======>>>>>> Maokai being untargetable again is a bug (someone probably accidentally committed an older version and reverted the change).

There's a page on the League of Legends wiki on untargetability : http://leagueoflegends.wikia.com/wiki/Untargetable
It should explain the details of untargetability to you. As for the whole "strike Maokai with skillshots whilst he is traveling" thing, that shouldn't be possible. Perhaps it's a glitch, but he probably has a small part at the beginning of his animation in which he is still targetable.
EDIT: Oh hey, I didn't know that Maokai lost his untargetability. In that case, ignore me! (someone should probably edit the untargetability page then though)

If this is the intention with Maokai, I don't suppose you would also be willing to clear up the confusion with other champions who become "untargetable." I'm mainly thinking about Elise, who is especially vulnerable to AOE during her Rappel because she is completely stationary while untargetable.

If this is also intended, it would be nice if you could update the tooltips of other such champions as well to state exact interactions, because it's not obvious but has a large impact on the strategic options for players using the champions.