I get this error and I think I understand it but I look at my code and the enemy does seem to be on the stage. The error is on the last line of the Enemy.as. It can't remove the enemy display object as it seems that the stage doesn't have it as a child.

Main.as (instantiates enemy:Enemy)

Enemy.as (holds the problem code) Basically it is saying that the stage doesn't hold the display object enemy.

I will post the parts where enemy is mentioned.

Main.as

private var enemy:Enemy = new Enemy(stage, hero);

...

for (var i:int=0; i< 5; i++)

                              {

                                        enemy = new Enemy(stage, hero);

                                        enemy.x = 100 + offsetEnemy;

                                        enemyVector[i] = enemy;//populate the vector with enemies.

                                        addChild(enemy);

                                        enemy.cacheAsBitmap = true;

                              }

Enemy.as

package

{

          import flash.display.MovieClip;

          import flash.display.Stage;

          import flash.events.Event;

          import flash.display.Stage;

          public class Enemy extends MovieClip

          {

                    private var stageRef:Stage;

                    private var vy:Number = 1; //y velocity

                    private var ay:Number = .1; //y acceleration

                    private var target:Hero;

                    public function Enemy(stageRef:Stage, target:Hero) : void

                    {

                              this.stageRef = stageRef;

                              this.target = target;

                              x = Math.random() * stageRef.stageWidth;

                              y = -5;

                              addEventListener(Event.ENTER_FRAME, loop, false, 0, true);

                    }

                    private function loop(e:Event) : void

                    {

                              vy += ay;

                              y += vy;

                              if (y > stageRef.stageHeight)

                                        removeSelf();

                    }

                    private function removeSelf() : void {

                              removeEventListener(Event.ENTER_FRAME, loop);

                              if (stageRef.contains(this))

                                        stageRef.removeChild(this);

                    }

          }

}