Bukkit

What steps will reproduce the problem?
starting the plugin up with vault on craftbukkit 1.2.4 R1.0

What is the expected output? What do you see instead?
I see this:
182 recipes
27 achievements
13:25:33 [INFO] Starting minecraft server version 1.2.4
13:25:33 [WARNING] **** NOT ENOUGH RAM!
13:25:33 [WARNING] To start the server with more ram, launch it as "java -Xmx102
4M -Xms1024M -jar minecraft_server.jar"
13:25:33 [INFO] Loading properties
13:25:33 [INFO] Starting Minecraft server on 192.168.1.5:25565
13:25:35 [INFO] This server is running CraftBukkit version git-Bukkit-1.2.4-R1.0
-b2126jnks (MC: 1.2.4) (Implementing API version 1.2.4-R1.0)
13:25:38 [SEVERE] Could not load 'plugins\Karma.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError
: me/icyrelic/com/Karma : Unsupported major.minor version 51.0
        at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.j
ava:150)
        at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.
java:305)
        at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager
.java:230)
        at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:207)
        at org.bukkit.craftbukkit.CraftServer.<init>(CraftServer.java:183)
        at net.minecraft.server.ServerConfigurationManager.<init>(ServerConfigur
ationManager.java:53)
        at net.minecraft.server.MinecraftServer.init(MinecraftServer.java:156)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:425)
        at net.minecraft.server.ThreadServerApplication.run(SourceFile:490)
Caused by: java.lang.UnsupportedClassVersionError: me/icyrelic/com/Karma : Unsup
ported major.minor version 51.0
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClassCond(Unknown Source)
        at java.lang.ClassLoader.defineClass(Unknown Source)
        at java.security.SecureClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.access$000(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:41)
        at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.
java:29)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Unknown Source)
        at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.j
ava:139)
        ... 8 more
13:25:42 [INFO] [PermissionsEx] sql backend registered!
13:25:42 [INFO] [PermissionsEx] file backend registered!
13:25:42 [INFO] [PermissionsEx] PermissionEx plugin initialized.
13:25:43 [SEVERE] null initializing War v1.7 (Doolittle) (Is it up to date?)
java.lang.NullPointerException
        at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:211)
        at org.bukkit.craftbukkit.CraftServer.<init>(CraftServer.java:183)
        at net.minecraft.server.ServerConfigurationManager.<init>(ServerConfigur
ationManager.java:53)
        at net.minecraft.server.MinecraftServer.init(MinecraftServer.java:156)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:425)
        at net.minecraft.server.ThreadServerApplication.run(SourceFile:490)
13:25:43 [INFO] [Vault] Loading Vault v1.2.13-b148

Fix: Bad JRE Version

What Version did you open the server with? The version of the plugin and the version that you started your server with has to be the same, as jre7 has more features so if you use jre6, those features don't exist meaning they will come up as an exception error. It even says there: Unsupported major.minor version 51.0 And 'java.lang.UnsupportedClassVersionError' Meaning '51.0' is a lower version than the server your running, or it is incompatible. Hope this helps, This error happened to me yesterday, and I found out the solution :D