What is the highest level you can get a single skill?

I always thought it was 16:

12 naturally.
3 with superior rune.
1 from headwear.

Occasionally you have weapons that have a chance to increase skills by +1 through use so maybe 15% of the time you can get that skill to a possible 17.

I ask this because "some guy" on a message board claims he has 19 skill in Shadow Arts. My initial thought is "another bragger on the net full of crap".

Still, it made me wonder the the highest max is and if I'm not missing some tactic the pros use to boost skills up higher.

i know you can pop a necro to 18 i think using the right skills (17 for sure)

EDIT

AWAKEN THE BLOOD GIVES +2 BLOOD AND +2 CURSES for 18

You can get up to 19 for curses and blood for a necro:

12 from att points
3 from rune
1 from heargear
+2 from awaken the blood (temporary)
+1 from off hand or staff mod

So 12+3+1+2+1=19

But, if your asking permanent, 16, then either 17-19 temporary or part of the time.

i forgot that thanks for the catch

Don't forget the temporary +1 you can buy from gods.

Eles can do this too with glyph of elemental power.