if you use tabpanel and have more than 3 tab,then when you put some component in the tabs,i put gridpanel to the tabpanel,when you active the last tab,then close first or second tab ,then close the last tab,you can find the content of last tab will be display at the first tab.
i trace the error and find OnRemove method:
Code:
onRemove : function(c){
        var te = Ext.get(c.tabEl);
        // check if the tabEl exists, it won't if the tab isn't rendered
        if(te){
            te.select('a').removeAllListeners();
            Ext.destroy(te);
        }
        Ext.TabPanel.superclass.onRemove.call(this, c);
        this.stack.remove(c);
        delete c.tabEl;
        c.un('disable', this.onItemDisabled, this);
        c.un('enable', this.onItemEnabled, this);
        c.un('titlechange', this.onItemTitleChanged, this);
        c.un('iconchange', this.onItemIconChanged, this);
        c.un('beforeshow', this.onBeforeShowItem, this);
        if(c == this.activeTab){
            var next = this.stack.next();
            if(next){
                this.setActiveTab(next);//here is the break-point
            }else if(this.items.getCount() > 0){
                this.setActiveTab(0);
            }else{
                this.setActiveTab(null);
            }
        }
        if(!this.destroying){
            this.delegateUpdates();
        }
and the setActiveTab method:
Code:
 setActiveTab : function(item){
        item = this.getComponent(item);
        if(this.fireEvent('beforetabchange', this, item, this.activeTab) === false){
            return;
        }
        if(!this.rendered){
            this.activeTab = item;
            return;
        }
        if(this.activeTab != item){
            if(this.activeTab){
                var oldEl = this.getTabEl(this.activeTab);
                if(oldEl){
                    Ext.fly(oldEl).removeClass('x-tab-strip-active');
                }
            }
            this.activeTab = item;
            if(item){
                var el = this.getTabEl(item);
                Ext.fly(el).addClass('x-tab-strip-active');
                this.stack.add(item);

                this.layout.setActiveItem(item);
                // Need to do this here, since setting the active tab slightly changes the size
                this.delegateUpdates();
                if(this.scrolling){
                    this.scrollToTab(item, this.animScroll);
                }
            }
            this.fireEvent('tabchange', this, item);
        }
    }
i think ,the active panel removed at OnRemove event,but SetActiveTab method use it,so,it cann't be destroy