Sadly, we can't really tell you anything about this error as the log just tells us you have a TON of mods and one or more of them add a entity using old ModLoader/Vanilla mechnics over the ID range that vanilla allows.

 

Only bet is to start kicking off mods untill you figure it out.

I can tell you one thing.

This:

this>>>    at net.minecraft.network.packet.Packet24MobSpawn.func_73279_a(SourceFile:87)

Indicates that the class was modified. The normal call location is something like: Packet24MobSpawn.java

 

So, someone edited (or used ASM rewrite on) the base class and created a bug. I dunno who and I do not see a full log.

Actually no, Unless some mod is explicitly removing the file name... 'SourceFile' is what Mojang's obfuscation sets it to.

He has a mod somewhere that is using a id > 127 thus being negative and can't be looked up.