We just had two TM1000 TimeMachine brand GPS NTP servers lose clock sync at the same time, in two different cities (LA and Santa Barbara). The  outage lasted about five minutes, during which the NTP servers were responding, but with time that was 1900 seconds out of sync. The devices showed satellite lock on 8 birds (not all the same ones). I've never seen this behavior before with years of NTP clock experience. 

It could be that these inexpensive NTP servers aren't very selective about bogus inputs, as I would have expected them to lose synch in the event of a GPS signal failure. Instead they produced garbage. Our PRTG NTP monitor logged the problem this way:

      

The target server did not return a valid time. To resolve this issue, use a packet analyzing tool and do a trace of the NTP packets to check if all fields are correctly populated. (code: PE085)