Didn't work correctly before. Now it does

src/net/minecraft/server/WorldServer.java

@@ -1,6 +1,5 @@
 package net.minecraft.server;
 
-
 import java.io.File;
 import java.util.Random;
 import java.util.ArrayList;
@@ -91,10 +90,16 @@ public class WorldServer extends World {
     // CraftBukkit start
     @Override
     public boolean a(int i1, int j1, int k1, int l1, boolean flag) {
-    	BlockCanBuildEvent event = new BlockCanBuildEvent(Type.BLOCK_CANBUILD, getWorld().getBlockAt(j1, k1, l1), super.a(i1, j1, k1, l1, flag));
+    	boolean superResult = super.a(i1, j1, k1, l1, flag); 
+    	
+    	if (!flag) {
+	    	BlockCanBuildEvent event = new BlockCanBuildEvent(Type.BLOCK_CANBUILD, getWorld().getBlockAt(j1, k1, l1), !superResult);
 	    	server.getPluginManager().callEvent(event);
 	    	
-    	return event.isCancelled();
+	    	return !event.isCancelled();
+    	} else {
+    		return superResult;
+    	}
     }
     // CraftBukkit stop