One weekend, I was at Boeing Field for a visiting warbird.  I was pleasantly surprised to see some US Marine Corps helicopters across the field too.  A combination of UH-1Y Venoms and AH-1Z Vipers were on the ramp.  I had no idea if or when they would fly.  However, luck was on my side as a Venom/Viper pair fired up and launched on a training mission.  The rest stayed on the ramp while I was there but this pair taxied out to the main runway and then departed past my location.  A nice extra!