I'm experiencing issues when on certain entries in the ender compendium. Whenever a player hits an Enderman, this occurs for the first time, and attempts to purchase the Enderman research will also cause an ISE. It also happened with some other entry that I couldnt test when i was flying around in The End. FML-Client-Latest.log
Specifically, it seems to be this:
[22:47:43] [Client thread/ERROR] [FML/]: Exception caught during firing event cpw.mods.fml.common.network.FMLNetworkEvent$ClientCustomPacketEvent@4c7c43c0: java.lang.NullPointerException at chylex.hee.mechanics.compendium.player.PlayerDiscoveryList.loadFromNBTList(PlayerDiscoveryList.java:36) ~[PlayerDiscoveryList.class:?] at chylex.hee.mechanics.compendium.player.PlayerCompendiumData.loadNBTData(PlayerCompendiumData.java:188) ~[PlayerCompendiumData.class:?] at chylex.hee.mechanics.compendium.player.PlayerCompendiumData.<init>(PlayerCompendiumData.java:45) ~[PlayerCompendiumData.class:?] at chylex.hee.packets.client.C19CompendiumData.read(C19CompendiumData.java:52) ~[C19CompendiumData.class:?] at chylex.hee.packets.PacketPipeline.readPacket(PacketPipeline.java:126) ~[PacketPipeline.class:?] at chylex.hee.packets.PacketPipeline.onClientPacketReceived(PacketPipeline.java:151) ~[PacketPipeline.class:?] at cpw.mods.fml.common.eventhandler.ASMEventHandler_898_PacketPipeline_onClientPacketReceived_ClientCustomPacketEvent.invoke(.dynamic) ~[?:?] at cpw.mods.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:54) ~[ASMEventHandler.class:?] at cpw.mods.fml.common.eventhandler.EventBus.post(EventBus.java:140) [EventBus.class:?] at cpw.mods.fml.common.network.FMLEventChannel.fireRead(FMLEventChannel.java:103) [FMLEventChannel.class:?] at cpw.mods.fml.common.network.NetworkEventFiringHandler.channelRead0(NetworkEventFiringHandler.java:30) [NetworkEventFiringHandler.class:?] at cpw.mods.fml.common.network.NetworkEventFiringHandler.channelRead0(NetworkEventFiringHandler.java:18) [NetworkEventFiringHandler.class:?] at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:98) [SimpleChannelInboundHandler.class:?] at io.netty.channel.DefaultChannelHandlerContext.invokeChannelRead(DefaultChannelHandlerContext.java:337) [DefaultChannelHandlerContext.class:?] at io.netty.channel.DefaultChannelHandlerContext.fireChannelRead(DefaultChannelHandlerContext.java:323) [DefaultChannelHandlerContext.class:?] at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:785) [DefaultChannelPipeline.class:?] at io.netty.channel.embedded.EmbeddedChannel.writeInbound(EmbeddedChannel.java:169) [EmbeddedChannel.class:?] at cpw.mods.fml.common.network.internal.FMLProxyPacket.func_148833_a(FMLProxyPacket.java:77) [FMLProxyPacket.class:?] at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:212) [ej.class:?] at net.minecraft.client.multiplayer.PlayerControllerMP.func_78765_e(PlayerControllerMP.java:273) [bje.class:?] at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1602) [bao.class:?] at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:973) [bao.class:?] at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:898) [bao.class:?] at net.minecraft.client.main.Main.main(SourceFile:148) [Main.class:?] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_91] at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_91] at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_91] at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_91] at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) [launchwrapper-1.12.jar:?] at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?] [22:47:43] [Client thread/ERROR] [FML/]: Index: 1 Listeners: [22:47:43] [Client thread/ERROR] [FML/]: 0: NORMAL [22:47:43] [Client thread/ERROR] [FML/]: 1: ASM: chylex.hee.packets.PacketPipeline@493cc03a onClientPacketReceived(Lcpw/mods/fml/common/network/FMLNetworkEvent$ClientCustomPacketEvent;)V [22:47:43] [Client thread/ERROR] [FML/]: NetworkEventFiringHandler exception java.lang.NullPointerException at chylex.hee.mechanics.compendium.player.PlayerDiscoveryList.loadFromNBTList(PlayerDiscoveryList.java:36) ~[PlayerDiscoveryList.class:?] at chylex.hee.mechanics.compendium.player.PlayerCompendiumData.loadNBTData(PlayerCompendiumData.java:188) ~[PlayerCompendiumData.class:?] at chylex.hee.mechanics.compendium.player.PlayerCompendiumData.<init>(PlayerCompendiumData.java:45) ~[PlayerCompendiumData.class:?] at chylex.hee.packets.client.C19CompendiumData.read(C19CompendiumData.java:52) ~[C19CompendiumData.class:?] at chylex.hee.packets.PacketPipeline.readPacket(PacketPipeline.java:126) ~[PacketPipeline.class:?] at chylex.hee.packets.PacketPipeline.onClientPacketReceived(PacketPipeline.java:151) ~[PacketPipeline.class:?] at cpw.mods.fml.common.eventhandler.ASMEventHandler_898_PacketPipeline_onClientPacketReceived_ClientCustomPacketEvent.invoke(.dynamic) ~[?:?] at cpw.mods.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:54) ~[ASMEventHandler.class:?] at cpw.mods.fml.common.eventhandler.EventBus.post(EventBus.java:140) ~[EventBus.class:?] at cpw.mods.fml.common.network.FMLEventChannel.fireRead(FMLEventChannel.java:103) ~[FMLEventChannel.class:?] at cpw.mods.fml.common.network.NetworkEventFiringHandler.channelRead0(NetworkEventFiringHandler.java:30) ~[NetworkEventFiringHandler.class:?] at cpw.mods.fml.common.network.NetworkEventFiringHandler.channelRead0(NetworkEventFiringHandler.java:18) ~[NetworkEventFiringHandler.class:?] at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:98) ~[SimpleChannelInboundHandler.class:?] at io.netty.channel.DefaultChannelHandlerContext.invokeChannelRead(DefaultChannelHandlerContext.java:337) [DefaultChannelHandlerContext.class:?] at io.netty.channel.DefaultChannelHandlerContext.fireChannelRead(DefaultChannelHandlerContext.java:323) [DefaultChannelHandlerContext.class:?] at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:785) [DefaultChannelPipeline.class:?] at io.netty.channel.embedded.EmbeddedChannel.writeInbound(EmbeddedChannel.java:169) [EmbeddedChannel.class:?] at cpw.mods.fml.common.network.internal.FMLProxyPacket.func_148833_a(FMLProxyPacket.java:77) [FMLProxyPacket.class:?] at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:212) [ej.class:?] at net.minecraft.client.multiplayer.PlayerControllerMP.func_78765_e(PlayerControllerMP.java:273) [bje.class:?] at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1602) [bao.class:?] at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:973) [bao.class:?] at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:898) [bao.class:?] at net.minecraft.client.main.Main.main(SourceFile:148) [Main.class:?] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_91] at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_91] at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_91] at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_91] at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) [launchwrapper-1.12.jar:?] at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?] [22:47:43] [Client thread/ERROR] [FML/]: There was a critical exception handling a packet on channel hee java.lang.NullPointerException at chylex.hee.mechanics.compendium.player.PlayerDiscoveryList.loadFromNBTList(PlayerDiscoveryList.java:36) ~[PlayerDiscoveryList.class:?] at chylex.hee.mechanics.compendium.player.PlayerCompendiumData.loadNBTData(PlayerCompendiumData.java:188) ~[PlayerCompendiumData.class:?] at chylex.hee.mechanics.compendium.player.PlayerCompendiumData.<init>(PlayerCompendiumData.java:45) ~[PlayerCompendiumData.class:?] at chylex.hee.packets.client.C19CompendiumData.read(C19CompendiumData.java:52) ~[C19CompendiumData.class:?] at chylex.hee.packets.PacketPipeline.readPacket(PacketPipeline.java:126) ~[PacketPipeline.class:?] at chylex.hee.packets.PacketPipeline.onClientPacketReceived(PacketPipeline.java:151) ~[PacketPipeline.class:?] at cpw.mods.fml.common.eventhandler.ASMEventHandler_898_PacketPipeline_onClientPacketReceived_ClientCustomPacketEvent.invoke(.dynamic) ~[?:?] at cpw.mods.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:54) ~[ASMEventHandler.class:?] at cpw.mods.fml.common.eventhandler.EventBus.post(EventBus.java:140) ~[EventBus.class:?] at cpw.mods.fml.common.network.FMLEventChannel.fireRead(FMLEventChannel.java:103) ~[FMLEventChannel.class:?] at cpw.mods.fml.common.network.NetworkEventFiringHandler.channelRead0(NetworkEventFiringHandler.java:30) ~[NetworkEventFiringHandler.class:?] at cpw.mods.fml.common.network.NetworkEventFiringHandler.channelRead0(NetworkEventFiringHandler.java:18) ~[NetworkEventFiringHandler.class:?] at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:98) ~[SimpleChannelInboundHandler.class:?] at io.netty.channel.DefaultChannelHandlerContext.invokeChannelRead(DefaultChannelHandlerContext.java:337) ~[DefaultChannelHandlerContext.class:?] at io.netty.channel.DefaultChannelHandlerContext.fireChannelRead(DefaultChannelHandlerContext.java:323) ~[DefaultChannelHandlerContext.class:?] at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:785) ~[DefaultChannelPipeline.class:?] at io.netty.channel.embedded.EmbeddedChannel.writeInbound(EmbeddedChannel.java:169) ~[EmbeddedChannel.class:?] at cpw.mods.fml.common.network.internal.FMLProxyPacket.func_148833_a(FMLProxyPacket.java:77) [FMLProxyPacket.class:?] at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:212) [ej.class:?] at net.minecraft.client.multiplayer.PlayerControllerMP.func_78765_e(PlayerControllerMP.java:273) [bje.class:?] at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1602) [bao.class:?] at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:973) [bao.class:?] at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:898) [bao.class:?] at net.minecraft.client.main.Main.main(SourceFile:148) [Main.class:?] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_91] at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_91] at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_91] at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_91] at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) [launchwrapper-1.12.jar:?] at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]
This does not crash a server, so it is not a high-priority issue, but an inconvenience that I can't seem to source.

Hmm... does it happen in singleplayer? If so, could you try removing LegacyCraft, plus any other suspicious mods that could cause the issue, and try if the issue keeps happening?

Do you have any other mod that modifies Endermen? Please post the hee/compendium.nbt file.

LegacyCraft might be modifying Endermen. I don't think anything else would be.
I cannot seem to find that file. The only folder named hee is in the world directory and it doesn't contain it.

It seems to be related to LegacyCraft's overrides on the endermen. Disabling Mob Overrides for them allows the entry to act normally.

Ok. Since mob overrides from other mods are not supported, I'm closing the issue - in HEE 2, this would trigger an integrity mismatch that prevents the game from running and reports which mod is overriding the mob.

I might look into porting it back to HEE 1, but people seem to be moving to 1.9 now which won't have an official update of HEE 1, so it's not a common issue.