In 5.12.1.5, it is stated that:

"The queue size is the maximum value supported by the Server when monitoring Events. In this case, the Server is responsible for the Event buffer. If Events are lost, an Event of the type EventQueueOverflowEventType is placed in the queue. This Event is generated when the first Event has to be discarded on a MonitoredItem subscribing for Events. It is put into the Queue of the MonitoredItem in addition to the size of the Queue defined for this MonitoredItem without discarding any other Event. If discardOldest is set to TRUE it is put at the beginning of the queue and is never discarded, otherwise at the end. An aggregating Server shall not pass on such an Event. It shall be handled like other connection error scenarios."

So it looks like aggregating servers are not supposed to generate events of type EventQueueOverflowEventType when a queue is full. However, the last sentence is too vague and does not clearly describe how the server should behave.