Disregard that, definitely need to duplicate.

worldguard-bukkit/src/main/java/com/sk89q/worldguard/bukkit/WorldGuardPlugin.java

@@ -20,7 +20,6 @@
 package com.sk89q.worldguard.bukkit;
 
 import com.google.common.collect.ImmutableList;
-import com.sk89q.bukkit.util.ClassSourceValidator;
 import com.sk89q.bukkit.util.CommandsManagerRegistration;
 import com.sk89q.minecraft.util.commands.CommandException;
 import com.sk89q.minecraft.util.commands.CommandPermissionsException;
@@ -60,6 +59,7 @@
 import com.sk89q.worldguard.bukkit.listener.WorldGuardWorldListener;
 import com.sk89q.worldguard.bukkit.listener.WorldRulesListener;
 import com.sk89q.worldguard.bukkit.session.BukkitSessionManager;
+import com.sk89q.worldguard.bukkit.util.ClassSourceValidator;
 import com.sk89q.worldguard.bukkit.util.Events;
 import com.sk89q.worldguard.commands.GeneralCommands;
 import com.sk89q.worldguard.commands.ProtectionCommands;
@@ -71,7 +71,6 @@
 import com.sk89q.worldguard.protection.managers.storage.RegionDriver;
 import com.sk89q.worldguard.protection.managers.storage.file.DirectoryYamlDriver;
 import com.sk89q.worldguard.protection.managers.storage.sql.SQLDriver;
-import com.sk89q.worldguard.protection.regions.ProtectedCuboidRegion;
 import com.sk89q.worldguard.protection.regions.ProtectedRegion;
 import com.sk89q.worldguard.util.logging.RecordMessagePrefixer;
 import org.bstats.bukkit.Metrics;
@@ -137,6 +136,10 @@ public static WorldGuardPlugin inst() {
      */
     @Override
     public void onEnable() {
+        // Catch bad things being done by naughty plugins that include WorldGuard's classes
+        ClassSourceValidator verifier = new ClassSourceValidator(this);
+        verifier.reportMismatches(ImmutableList.of(WorldGuard.class, ProtectedRegion.class, Flag.class));
+
         configureLogger();
 
         getDataFolder().mkdirs(); // Need to create the plugins/WorldGuard folder
@@ -150,13 +153,6 @@ public void onEnable() {
         // Set the proper command injector
         commands.setInjector(new SimpleInjector(WorldGuard.getInstance()));
 
-        // Catch bad things being done by naughty plugins that include WorldGuard's classes
-        try {
-            ClassSourceValidator verifier = new ClassSourceValidator(this);
-            verifier.reportMismatches(ImmutableList.of(WorldGuard.class, ProtectedRegion.class, Flag.class));
-        } catch (NoClassDefFoundError ignored) { // was added to WE two years ago but who knows
-        }
-
         // Register command classes
         final CommandsManagerRegistration reg = new CommandsManagerRegistration(this, commands);
         reg.register(ToggleCommands.class);

worldguard-bukkit/src/main/java/com/sk89q/worldguard/bukkit/util/ClassSourceValidator.java

@@ -0,0 +1,164 @@
+/*
+ * WorldGuard, a suite of tools for Minecraft
+ * Copyright (C) sk89q <http://www.sk89q.com>
+ * Copyright (C) WorldGuard team and contributors
+ *
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License
+ * for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.sk89q.worldguard.bukkit.util;
+
+import com.google.common.base.Strings;
+import com.google.common.collect.ImmutableMap;
+import org.bukkit.Bukkit;
+import org.bukkit.plugin.Plugin;
+import org.bukkit.plugin.java.PluginClassLoader;
+
+import javax.annotation.Nullable;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+
+/**
+ * Validates that certain specified classes came from the same source as
+ * a plugin.
+ * This is copied from the same class in WorldEdit because unfortunately
+ * trying to use WorldEdit's means we're susceptible to getting a bad version
+ * of this class if another plugin shades it....which is exactly what we're
+ * trying to detect and report.
+ */
+public class ClassSourceValidator {
+
+    private static final String SEPARATOR_LINE = Strings.repeat("*", 46);
+    private static final Method loadClass;
+
+    static {
+        Method tmp;
+        try {
+            tmp = PluginClassLoader.class.getDeclaredMethod("loadClass0",
+                    String.class, boolean.class, boolean.class, boolean.class);
+            tmp.setAccessible(true);
+        } catch (NoSuchMethodException e) {
+            tmp = null;
+        }
+        loadClass = tmp;
+    }
+
+    private final Plugin plugin;
+    @Nullable
+    private final ClassLoader expectedClassLoader;
+
+    /**
+     * Create a new instance.
+     *
+     * @param plugin The plugin
+     */
+    public ClassSourceValidator(Plugin plugin) {
+        checkNotNull(plugin, "plugin");
+        this.plugin = plugin;
+        this.expectedClassLoader = plugin.getClass().getClassLoader();
+    }
+
+    /**
+     * Return a map of classes that been loaded from a different source.
+     *
+     * @param classes A list of classes to check
+     * @return The results
+     */
+    public Map<Class<?>, Plugin> findMismatches(List<Class<?>> classes) {
+        checkNotNull(classes, "classes");
+
+        if (expectedClassLoader == null || loadClass == null) {
+            return ImmutableMap.of();
+        }
+
+        Map<Class<?>, Plugin> mismatches = new HashMap<>();
+
+        for (Plugin target : Bukkit.getPluginManager().getPlugins()) {
+            if (target == plugin) {
+                continue;
+            }
+            ClassLoader targetLoader = target.getClass().getClassLoader();
+            if (!(targetLoader instanceof PluginClassLoader)) {
+                continue;
+            }
+            for (Class<?> testClass : classes) {
+                Class<?> targetClass;
+                try {
+                    targetClass = (Class<?>) loadClass.invoke(targetLoader, testClass.getName(), false, false, false);
+                } catch (IllegalAccessException | InvocationTargetException ignored) {
+                    continue;
+                }
+                if (targetClass.getClassLoader() != expectedClassLoader) {
+                    mismatches.putIfAbsent(testClass, targetClass.getClassLoader() == targetLoader ? target : null);
+                }
+            }
+        }
+
+        return mismatches;
+    }
+
+    /**
+     * Reports classes that have come from a different source.
+     *
+     * <p>The warning is emitted to the log.</p>
+     *
+     * @param classes The list of classes to check
+     */
+    public void reportMismatches(List<Class<?>> classes) {
+        if (Boolean.getBoolean("enginehub.disable.class.source.validation")) {
+            return;
+        }
+        Map<Class<?>, Plugin> mismatches = findMismatches(classes);
+
+        if (mismatches.isEmpty()) {
+            return;
+        }
+        StringBuilder builder = new StringBuilder("\n");
+
+        builder.append(SEPARATOR_LINE).append("\n");
+        builder.append("** /!\\    SEVERE WARNING    /!\\\n");
+        builder.append("** \n");
+        builder.append("** A plugin developer has included a portion of \n");
+        builder.append("** ").append(plugin.getName()).append(" into their own plugin, so rather than using\n");
+        builder.append("** the version of ").append(plugin.getName()).append(" that you downloaded, you\n");
+        builder.append("** will be using a broken mix of old ").append(plugin.getName()).append(" (that came\n");
+        builder.append("** with the plugin) and your downloaded version. THIS MAY\n");
+        builder.append("** SEVERELY BREAK ").append(plugin.getName().toUpperCase(Locale.ROOT)).append(" AND ALL OF ITS FEATURES.\n");
+        builder.append("**\n");
+        builder.append("** This may have happened because the developer is using\n");
+        builder.append("** the ").append(plugin.getName()).append(" API and thinks that including\n");
+        builder.append("** ").append(plugin.getName()).append(" is necessary. However, it is not!\n");
+        builder.append("**\n");
+        builder.append("** Here are some files that have been overridden:\n");
+        builder.append("** \n");
+        for (Map.Entry<Class<?>, Plugin> entry : mismatches.entrySet()) {
+            Plugin badPlugin = entry.getValue();
+            String url = badPlugin == null
+                    ? "(unknown)"
+                    : badPlugin.getName() + " (" + badPlugin.getClass().getProtectionDomain().getCodeSource().getLocation() + ")";
+            builder.append("** '").append(entry.getKey().getSimpleName()).append("' came from '").append(url).append("'\n");
+        }
+        builder.append("**\n");
+        builder.append("** Please report this to the plugins' developers.\n");
+        builder.append(SEPARATOR_LINE).append("\n");
+
+        plugin.getLogger().severe(builder.toString());
+    }
+}