In betalabs (also found on plwiki wmf.19) on RC page in 'Search filter area' enter search criteria that return 'No Filters found'. That will display 'TypeError: undefined is not an object (evaluating 'filterItem.isSelected')' in the Console.
Uncaught TypeError: Cannot read property 'isSelected' of undefined at MwRcfiltersController.mw.rcfilters.Controller.toggleFilterSelect (eval at <anonymous> at MwRcfiltersUiFilterWrapperWidget.mw.rcfilters.ui.FilterWrapperWidget.onTextInputEnter at OoUiTextInputWidget.oo.EventEmitter.emit (eval at <anonymous> at OoUiTextInputWidget.OO.ui.TextInputWidget.onKeyPress at HTMLInputElement.dispatch at HTMLInputElement.elemData.handle