Bukkit

2012-11-12 16:05:52 [INFO] [UsefulVillagers] Disabling UsefulVillagers v0.4.0
2012-11-12 16:05:52 [SEVERE] Error occurred while disabling UsefulVillagers v0.4.0 (Is it up to date?)
java.lang.NoClassDefFoundError: com/google/gson/GsonBuilder
    at org.randomgd.bukkit.workers.WorkerHandler.serializeVillagerData(WorkerHandler.java:441)
    at org.randomgd.bukkit.workers.WorkerHandler.onDisable(WorkerHandler.java:171)
    at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:219)
    at org.bukkit.plugin.java.JavaPluginLoader.disablePlugin(JavaPluginLoader.java:400)
    at org.bukkit.plugin.SimplePluginManager.disablePlugin(SimplePluginManager.java:400)
    at org.bukkit.plugin.SimplePluginManager.disablePlugins(SimplePluginManager.java:393)
    at org.bukkit.craftbukkit.CraftServer.disablePlugins(CraftServer.java:295)
    at net.minecraft.server.MinecraftServer.stop(MinecraftServer.java:350)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:460)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:818)
Caused by: java.lang.ClassNotFoundException: com.google.gson.GsonBuilder
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:44)
    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:29)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    ... 10 more

This is odd. You don't have any problem at start-up, but it crashes at shutdown was the exact same class ... I'll investigate on that. It can be due to a wrong version of GSON. What version do you use ?