This event occurs when the WebDriver comes across an exception. For instance, if you try to search for a WebElement using findElement(), and that element doesn't exist on the page, the driver throws an exception (NoSuchElementException). At this point, this event is triggered, and the following method gets notified:

public void onException(java.lang.Throwable throwable, WebDriver driver)

In all the after<<event>> methods, we have seen that they will not be invoked if the driver comes across any exception. In that case, instead of those after<<event>> methods, the onException() method is invoked and the throwable object and the WebDriver object are sent to it as parameters.