I found that there is a bug if you have a cf cable in you myst age and relog there (not tested in overwold/nether/end). This Can also happen if you travel to that age and will cause this crash:
39 mods loaded
Optifine OptiFine_1.2.5_HD_C6
Minecraft Forge 3.3.8.152
FML v2.2.78.153
Forge Mod Loader version 2.2.78.153 for Minecraft 1.2.5
mod_CodeChickenCore : Available (minecraft.jar)
mod_MinecraftForge : Available (minecraft.jar)
mod_NotEnoughItems : Available (minecraft.jar)
mod_IRCMod : Available ([1.2.5] IRCMod.zip)
mod_IC2 : Available (industrialcraft-2-client.jar)
mod_AdvancedMachines : Available (AdvancedMachinesClient v1.5.zip)
mod_BuildCraftCore : Available (buildcraft-client-A-core-3.1.5.zip)
mod_BuildCraftEnergy : Available (buildcraft-client-B-energy-3.1.5.zip)
mod_BuildCraftFactory : Available (buildcraft-client-B-factory-3.1.5.zip)
mod_BuildCraftTransport : Available (buildcraft-client-B-transport-3.1.5.zip)
mod_BCIC2Crossover : Available (BCIC2crossover.IB.v0.08.zip)
mod_AdditionalBuildcraftObjects : Available (buildcraft-C-additionalbuildcraftobjects-f301@75bc315-94.zip)
mod_BuildCraftBuilders : Available (buildcraft-client-B-builders-3.1.5.zip)
mod_BuildCraftSilicon : Available (buildcraft-client-C-silicon-3.1.5.zip)
mod_AdditionalPipes : Available (buildcraft-client-DA-additionalpipes-3.0.10.zip)
mod_ComputerCraft : Available (ComputerCraft)
mod_RedPowerMachine : Available (RedPowerMachine-2.0pr5b2.zip)
mod_ccSensors : Available (ccSensors-client-MC125-B017pr1.zip)
mod_ClayMan : Available (ClaySoldiersMod_v62.zip)
mod_CCTurtle : Available (ComputerCraft)
mod_EE : Available (EE2ModV1.4.4.0.jar)
mod_Forestry : Available (forestry-client-A-1.4.4.2.jar)
mod_BuildCraftZFP : Available (forestry-client-A-1.4.4.2.jar)
mod_IC2NuclearControl : Available (IC2NuclearControl_client_v1.1.9b.zip)
mod_PigbearLaser : Available (Laser Mod 1.5.zip)
mod_LogisticsPipes : Available (LogisticsPipes-BC3-0.2.4A.zip)
mod_CompactSolars : Available (mod_compactsolars-client-2.2.0.5.zip)
mod_IronChest : Available (mod_ironchests-client-3.3.1.21.zip)
mod_ModularForceFieldSystem : Available (modularForceFieldSystemV1.2.5Beta6pre3_2_Client.zip)
mod_PortalGun : Available (portalgun)
mod_Railcraft : Available (Railcraft_Client_5.0.3.zip)
mod_RedPowerControl : Available (RedPowerControl-2.0pr5b2.zip)
mod_RedPowerCore : Available (RedPowerCore-2.0pr5b2.zip)
mod_RedPowerLogic : Available (RedPowerLogic-2.0pr5b2.zip)
mod_RedPowerWiring : Available (RedPowerWiring-2.0pr5b2.zip)
mod_RedPowerWorld : Available (RedPowerWorld-2.0pr5b2.zip)
mod_WirelessRedstoneCore : Available (WR-CBE Core-Client 1.2.0.zip)
mod_WirelessRedstoneAddons : Available (WR-CBE Addons-Client 1.2.1.zip)
mod_WirelessRedstoneRedPower : Available (WR-CBE RedPower-Client 1.2.1.1.zip)
Minecraft has crashed!
----------------------
Minecraft has stopped running because it encountered a problem.
--- BEGIN ERROR REPORT ef33ccc6 --------
Generated 20.07.12 17:49
Minecraft: Minecraft 1.2.5
OS: Windows 7 (amd64) version 6.1
Java: 1.7.0, Oracle Corporation
VM: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
LWJGL: 2.4.2
OpenGL: AMD Radeon HD 6310 Graphics version 4.1.10429 Compatibility Profile Context, ATI Technologies Inc.
java.lang.NullPointerException
at ic2.common.TileEntityCable.onCreated(TileEntityCable.java:90)
at ic2.common.TileEntityBlock$1.tickCallback(TileEntityBlock.java:27)
at mod_IC2.OnTickInGame(mod_IC2.java:1377)
at ic2.platform.Ic2.onTickInGame(Ic2.java:347)
at BaseMod.doTickInGame(BaseMod.java:48)
at cpw.mods.fml.common.modloader.BaseModTicker.sendTick(BaseModTicker.java:95)
at cpw.mods.fml.common.modloader.BaseModTicker.tickBaseMod(BaseModTicker.java:79)
at cpw.mods.fml.common.modloader.BaseModTicker.tickEnd(BaseModTicker.java:55)
at cpw.mods.fml.common.FMLCommonHandler.tickEnd(FMLCommonHandler.java:127)
at cpw.mods.fml.client.FMLClientHandler.onRenderTickEnd(FMLClientHandler.java:281)
at net.minecraft.client.Minecraft.x(Minecraft.java:875)
at net.minecraft.client.Minecraft.run(Minecraft.java:750)
at java.lang.Thread.run(Unknown Source)
--- END ERROR REPORT e86f263d ----------
It will also cause the chunks to reset.
I use Forge 152, IC2 .97 (I think) and Mystcraft 0.8.6b