[Bug] Ghost block on skyland : crop circle ? :O

Hello community
I've found a weird bug on my server, and i dont understand what does that, so i ask it here, maybe someone else already found it.
First, my plugin list with /plugins :
PistonJump, Groupmanager, statistician, voxelsniper, vault, realplugin, morephysics piston standalone, multiverse core, dragonnoportal, realshop2, shopkeepers, realshopping, machinacore, disguisecraft, machinafactory, openinv, hoverpad, angelstarterchest, machinabuilder, machinapump, jobs, citizens2, nocheatplus, worldedit, multiverse inventory, skript, alchemist, logblock, worldguard, casinoslots, quantumconnectors, bookmanager, iconomy, muddersmilk, craftbukkituptodate, herstronghold, gods, clearlag, simplespleef, falsebookcore, ichat, autoannouncer, essentials, no smoking, machinadrill, machinaredstonebridge, multiverse portals, DAC, scriptblock, dynmap, dynmap worldguard, machinaplanter, magicspells, denizen, falsebookIC, essentialschat, falsebookextra, mobarena, weatherman, blacksmith, essentialsprotect, falsebookblock, essentialsspawn, vanishnopacket, LWC, worldborder, homespawnplus, physicalshop, simpleclans, deathTPplus, dynmap simpleclans.

Pretty impressive list of plugins i know we are in a dedicated server with enough RAM so dont worry for that
My bug is that we have weird plate of ghost block on the 0 depth...
sometimes this is a half circle, sometimes its squares, with lapis lazuli or cobblestone or bedrock, ive even found a lapis lazuli block oO (not ore)

I post some screenshoot for you to see what it is :
http://img338.imageshack.us/img338/8449/25279099.png
http://img819.imageshack.us/img819/7139/72141255.png
http://img228.imageshack.us/img228/1246/84863723.png

Ive made some test, i can break a bloc, but when i do this, my friend dont see the block disapear. So this is client side, the block arent really here, when i do a worldedit selection and replace block, he say that there is 0 block changed, when i do a /repl 0 with a tool, i clic on the ghost block, and he say that the block selected is AIR...
So, this block are not really here...

Aliens ? Ghost ? or maybe the return of the nazi from the moon ?? I dont understand xD
Thanks for your help everyone

ps: sorry for my bad english, im frenshy, cheese eater xD

 

Just remove it, via voxel sniper, world edit ect.

 

i wouldve said its anti xray, but cant find any on the list. i dont see why the client would get just these IDs, so i think it may have to do with a plugin.