You can indicate that an error occurred using haptic feedback. It can be useful for long-running tasks that fail, like page loads or downloads. But you can also use it as errors occur like Twitter does when reaching the character limit.

Twitter app in the compose tweet screen. The character limit has been reached. The character limit indicator now shows 0 characters left, the circular progress indicator is complete, and it turns red in the app. It also produces a haptic feedback vibration that indicates an error. This is achieved by notifying that an error occurred to a UINotificationFeedbackGenerator object.