The spec says

Otherwise, the exception aborts processing of the event. No other observer methods of that event will be called. The
BeanManager.fireEvent() or Event.fire() method rethrows the exception. If the exception is a checked exception,
it is wrapped and rethrown as an (unchecked) ObserverException.

The JavaDoc for BM.fireEvent() and Event.fire() only mentions IllegalArgumentException. The fact that the ObserverException or any other unchecked exception raised by an observer method is rethrown should be mentioned.