It's not clear to me where the problem lies, but there is a problem with the combination of TreeCapitator 1.6.2.r01 (the current version for Minecraft 1.6.2) and Forge ModLoader build 830 and higher.  Up to build 829, everything works perfectly.  With build 830 and above, TreeCapitator silently fails to load, and any attempt to break a tree block results in an internal server error and disconnect.  The server logs this:

 

>2013-08-19 13:24:04 [sEVERE] [ForgeModLoader] A critical server error occured handling a packet, kicking 1120
java.lang.NullPointerException
        at net.minecraft.item.ItemInWorldManager.func_73079_d(ItemInWorldManager.java:336)
        at net.minecraft.item.ItemInWorldManager.func_73084_b(ItemInWorldManager.java:474)
        at net.minecraft.item.ItemInWorldManager.func_73082_a(ItemInWorldManager.java:295)
        at net.minecraft.network.NetServerHandler.func_72510_a(NetServerHandler.java:779)
        at net.minecraft.network.packet.Packet14BlockDig.func_73279_a(Packet14BlockDig.java:67)
        at net.minecraft.network.TcpConnection.func_74428_b(TcpConnection.java:464)
        at net.minecraft.network.NetServerHandler.func_72570_d(NetServerHandler.java:231)
        at net.minecraft.network.NetworkListenThread.func_71747_b(NetworkListenThread.java:54)
        at net.minecraft.server.dedicated.DedicatedServerListenThread.func_71747_b(DedicatedServerListenThread.java:34)
        at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:872)
        at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:318)
        at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:741)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:625)
        at net.minecraft.server.ThreadMinecraftServer.run(ThreadMinecraftServer.java:16)
13:24:04 [WARNING] Failed to handle packet for CaerMaster/10.24.32.20: java.lang.NullPointerException
java.lang.NullPointerException
        at net.minecraft.item.ItemInWorldManager.func_73079_d(ItemInWorldManager.java:336)
        at net.minecraft.item.ItemInWorldManager.func_73084_b(ItemInWorldManager.java:474)
        at net.minecraft.item.ItemInWorldManager.func_73082_a(ItemInWorldManager.java:295)
        at net.minecraft.network.NetServerHandler.func_72510_a(NetServerHandler.java:779)
        at net.minecraft.network.packet.Packet14BlockDig.func_73279_a(Packet14BlockDig.java:67)
        at net.minecraft.network.TcpConnection.func_74428_b(TcpConnection.java:464)
        at net.minecraft.network.NetServerHandler.func_72570_d(NetServerHandler.java:231)
        at net.minecraft.network.NetworkListenThread.func_71747_b(NetworkListenThread.java:54)
        at net.minecraft.server.dedicated.DedicatedServerListenThread.func_71747_b(DedicatedServerListenThread.java:34)
        at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:872)
        at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:318)
        at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:741)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:625)
        at net.minecraft.server.ThreadMinecraftServer.run(ThreadMinecraftServer.java:16)
13:24:04 [iNFO] §eCaerMaster left the game.

Cpw changed how coremod-normalmod hybrids are loaded. {The coremod containing a normal mod was never intended}

treecapitator will neeed to update.