Objective-C: Why I am getting exception trying to remove object from nsmutable array?

I thought what I am trying to do is really basic stuffs, I have 2 NSMutableArrays that contains MyItem objects. I want to remove the item from one array and add it to another array but it throws exception when trying to remove it and Yes to save you a complain, my understanding of fundamental OOP is a failure. I am not even dealing with enumerate thru array at all.I thought what I am trying to do is really basi