HBASE-15137 we made it so that CQTBE causes preemptive fast fail exception on the client.
It seems those 2 load control mechanists don't exactly align here. Server throws CallQueueTooBigException, CallDroppedException (from deadline scheduler) when it feels overloaded. Client should accept that behavior and retry. When servers sheds the load, and client also bails out, the load shedding bubbles up too high and high level impact on the client applications seems worse with PFFE turned on then without.