Now throwing expection if more than two arguments are passed to extend() when implementing

lib/class.js

@@ -361,6 +361,15 @@ function createImplement( base, ifaces, cname )
                 ext_base = args.pop()
             ;
 
+            // if any arguments remain, then they likely misunderstood what this
+            // method does
+            if ( args.length > 0 )
+            {
+                throw Error(
+                    "Expecting no more than two arguments for extend()"
+                );
+            }
+
             // if a base was already provided for extending, don't allow them to
             // give us yet another one (doesn't make sense)
             if ( base && ext_base )

test/test-class-implement.js

@@ -232,3 +232,16 @@ var Type = Interface.extend( {
     );
 } )();
 
+
+/**
+ * If more than two arguments are given to extend(), then the developer likely
+ * does not understand the API. Throw an error to prevent some bugs/confusion.
+ */
+( function testThrowsExceptionIfExtendContainsTooManyArguments()
+{
+    assert.throws( function()
+    {
+        Class.implement( Type ).extend( PlainFoo, {}, 'extra' );
+    }, Error, "extend() after implementing accepts no more than two args" );
+} )();
+