Pressing the escape key while the modal is visible will dismiss the modal without triggering the action associated with the button handling dismiss: 'modal'. That same is true for clicking the close button x in the modal header.
For example, when a cancel button is created in the controller using Ember.Object.create({title: 'Cancel', clicked:"cancel", dismiss: 'modal'}) clicking the button will trigger the cancel action and dismiss the modal. However, pressing the escape key or clicking the close button x in the modal header will only dismiss the modal without triggering the cancel action.
The examples in the Modal Showcase exhibit the behavior described above.