Another way to enter ground resonance with every thing working normal.
We were operating off the back end of an icebreaker with a Sikorsky HO3S on tricycle gear and a Bell 47 on floats.
The Sikorsky had three blades and with the cold you could not be sure that the blades were in phase during start-up. Because the Sikorsky only had parking brakes (which were not very effective) we had to keep the chocks in place and several cable tie-downs attached. Once the clutch was engaged the pilot would bring on the throttle and when he hit 100% NR we would release the tie-down and the pilot would take off. Of course this would only be when we were in rough seas or breaking ice as the ship would really roll. This rolling moment would perturb the rotor system and the helicopter would start working the shock struts.
Although the Bell would not normally go into ground resonance the rolling of the ship would perturb the spinning rotor and the helicopter would start working on the floats. To minimize this from happening we would hold the blades in the neutral flap position to facilitate the run-up of the blades.
One word of caution when the blade is snapped out of your fingers get your hand out of the way FAST, as the next blade will hit you. That was over fifty years ago and my hand still hurts.