Bukkit

What steps will reproduce the problem?
1. I do not know what happened, the server was running perfectly when this happened.
2.
3.

What is the expected output? What do you see instead?
I did not even touch MobDisguise when this happened.

What version of the product are you using?
1.81

Do you have an error log of what happened?
---- Minecraft Crash Report ----
// Surprise! Haha. Well, this is awkward.

Time: 8/9/12 2:17 PM
Description: Exception in server tick loop

java.lang.NullPointerException
    at net.minecraft.server.DedicatedServer.ah(DedicatedServer.java:244)
    at net.minecraft.server.DedicatedServer.a(DedicatedServer.java:192)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:415)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:539)
Relevant Details:
- Minecraft Version: 1.3.1
- Operating System: Windows 7 (amd64) version 6.1
- Java Version: 1.7.0_05, Oracle Corporation
- Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
- Memory: 8060243536 bytes (7686 MB) / 8232042496 bytes (7850 MB) up to 8232042496 bytes (7850 MB)
- JVM Flags: 2 total; -Xms8192M -Xmx8192M
- Type: Dedicated Server
- Is Modded: Definitely; 'craftbukkit'
- Profiler Position: N/A (disabled)

Please provide any additional information below.
This showed up in McMyAdmin:
15:12:36 CONSOLE: [WARNING] Task of 'MobDisguise' generated an exception
15:12:36 CONSOLE: [SEVERE] Could not pass event EntityDamageByEntityEvent to MobDisguise
15:12:36 CONSOLE: java.lang.NoSuchMethodError: net.minecraft.server.Packet29DestroyEntity.<init>(I)V