I've had this error message after a saving chunks crash twice, both times while flying around my world, but in different worlds. However, I am able to return to those worlds and continue flying around without triggering the error again. As you can see, I have a number of mods loaded and it could be a conflict, but as it lists IC2 in the bug report I figured I should post it here first.
Display Spoiler
Mods loaded: 28
ModLoader 1.1
mod_ModLoaderMp 1.0.0
mod_TwilightForest 1.6r2
mod_Somnia v18 [1.0.0]
mod_BuildCraftCore 3.1.3
mod_BuildCraftBuilders 3.1.3
mod_BuildCraftEnergy 3.1.3
mod_BuildCraftFactory 3.1.3
mod_BuildCraftTransport 3.1.3
mod_BuildCraftSilicon 3.1.3
mod_CraftGuide 1.4.0
mod_EE Equivalent Exchange 6.0.0
mod_Forestry 1.3.2.0
mod_BuildCraftZFP (Forestry Package)
mod_IC2 v1.64
mod_Millenaire 2.1.6
mod_mocreatures v3.2.2 (MC 1.1)
mod_Fossil Mod FOSSIL / ARCHEOLOGY v5.7 Engaged
mod_MCA Minecraft Comes Alive v1.2
mod_Pambonecraft 1.0.0
mod_Pamclothcraft 1.1
mod_Pamdesertcraft 1.1
mod_Pamfood 1.1
mod_PlasmaCraft 1.1.0/0.2.6
mod_QuickGuide 1.1
mod_stilts Beta 1.0.0 - By Nandonalt
mod_Timber 1.1.0
Balkon's WeaponMod 1.1.0 v8.3.0 Forge Edition
Minecraft has crashed!
----------------------
Minecraft has stopped running because it encountered a problem.
If you wish to report this, please copy this entire text and email it to support@mojang.com.
Please include a description of what you did when the error occured.
--- BEGIN ERROR REPORT 466ebcec --------
Generated 2/1/12 12:55 PM
Minecraft: Minecraft 1.1
OS: Windows 7 (amd64) version 6.1
Java: 1.6.0_21, Sun Microsystems Inc.
VM: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Sun Microsystems Inc.
LWJGL: 2.4.2
OpenGL: ATI Radeon HD 4800 Series version 3.3.10524 Compatibility Profile Context, ATI Technologies Inc.
java.lang.NullPointerException
at ic2.common.BlockTex.a(BlockTex.java:48)
at oe.getBlockDropped(Block.java:1033)
at oe.a(Block.java:437)
at aef.a(Explosion.java:166)
at vq.a(World.java:1938)
at vq.a(World.java:1929)
at aeg.l(SourceFile:83)
at aeg.B_(SourceFile:71)
at vq.a(World.java:1672)
at vq.f(World.java:1647)
at vq.m(World.java:1545)
at net.minecraft.client.Minecraft.k(SourceFile:1594)
at net.minecraft.client.Minecraft.x(SourceFile:709)
at net.minecraft.client.Minecraft.run(SourceFile:662)
at java.lang.Thread.run(Unknown Source)
--- END ERROR REPORT 1d39975e ----------