Noticed it from Automagy's vishroom soup, which poisons you in addition to other things. Thought it was on their end, but then I got the same crash while fighting some wither skeletons.

Ooops! Crashlog:

---- Minecraft Crash Report ----
// Why is it breaking :(

Time: 3/12/15 6:26 PM
Description: Ticking player

java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)
at java.util.HashMap$KeyIterator.next(HashMap.java:1453)
at net.minecraft.entity.EntityLivingBase.func_70679_bo(EntityLivingBase.java:521)
at net.minecraft.entity.EntityLivingBase.func_70030_z(EntityLivingBase.java:293)
at net.minecraft.entity.Entity.func_70071_h_(Entity.java:318)
at net.minecraft.entity.EntityLivingBase.func_70071_h_(EntityLivingBase.java:1561)
at net.minecraft.entity.player.EntityPlayer.func_70071_h_(EntityPlayer.java:288)
at net.minecraft.entity.player.EntityPlayerMP.func_71127_g(EntityPlayerMP.java:295)
at net.minecraft.network.NetHandlerPlayServer.func_147347_a(NetHandlerPlayServer.java:303)
at net.minecraft.network.play.client.C03PacketPlayer.func_148833_a(SourceFile:137)
at net.minecraft.network.play.client.C03PacketPlayer.func_148833_a(SourceFile:8)
at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:212)
at net.minecraft.network.NetworkSystem.func_151269_c(NetworkSystem.java:165)
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:659)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:547)
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:111)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:427)
at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685)

A detailed walkthrough of the error, its code path and all known details is as follows:

-- Head --
Stacktrace:
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)
at java.util.HashMap$KeyIterator.next(HashMap.java:1453)
at net.minecraft.entity.EntityLivingBase.func_70679_bo(EntityLivingBase.java:521)
at net.minecraft.entity.EntityLivingBase.func_70030_z(EntityLivingBase.java:293)
at net.minecraft.entity.Entity.func_70071_h_(Entity.java:318)
at net.minecraft.entity.EntityLivingBase.func_70071_h_(EntityLivingBase.java:1561)
at net.minecraft.entity.player.EntityPlayer.func_70071_h_(EntityPlayer.java:288)

-- Player being ticked --
Details:
Entity Type: null (net.minecraft.entity.player.EntityPlayerMP)
Entity ID: 333
Entity Name: Azumarill
Entity's Exact location: 28.12, 60.00, -94.54
Entity's Block location: World: (28,60,-95), Chunk: (at 12,3,1 in 1,-6; contains blocks 16,0,-96 to 31,255,-81), Region: (0,-1; contains chunks 0,-32 to 31,-1, blocks 0,0,-512 to 511,255,-1)
Entity's Momentum: 0.00, -0.08, 0.00
Stacktrace:
at net.minecraft.entity.player.EntityPlayerMP.func_71127_g(EntityPlayerMP.java:295)
at net.minecraft.network.NetHandlerPlayServer.func_147347_a(NetHandlerPlayServer.java:303)
at net.minecraft.network.play.client.C03PacketPlayer.func_148833_a(SourceFile:137)
at net.minecraft.network.play.client.C03PacketPlayer.func_148833_a(SourceFile:8)
at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:212)

-- Ticking connection --
Details:
Connection: net.minecraft.network.NetworkManager@172faad
Stacktrace:
at net.minecraft.network.NetworkSystem.func_151269_c(NetworkSystem.java:165)
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:659)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:547)
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:111)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:427)
at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685)

Hmm, I've actually experienced that error before myself. I might have found a fixed for it though. It will be in the next version.

This issue still happens in 1.1.6

Crash report:
http://pastebin.com/BAis5ukb

It usually happens when I get wither or poison.

I need details on your equipment.

Here the equipment i have:

• Armor:
  • Elementium Helmet of Revealing (Botania)
  • Elementium Chestplate (Botania)
  • Elementium Leggings (Botania)
  • Elementium Boots (Botania)
• Baubles:
  • Crimson Pendant (Botania)
  • Charged Ring of Shielding (Thaumcraft)
  • Ring of Nutrition (Forbidden Magic)
  • Girdle of Runic Shielding (Thaumcraft)
• Traveller's Gear:
  • Steadfast Spaulders (Witching Gadgets)
  • Berserker's Bracers (Witching Gadgets)
  • Wolven Cloak (Witching Gadgets)
  • Title: Crimson Cult (Witching Gadgets)