Ever hear of a phenomenon in helicopters called "ground resonance"?
https://en.wikipedia.org/wiki/Ground_resonance
It only happens to 'copters with articulated rotors and some sort of impact damping system on the landing gear (pneumatic wheels, oleo struts, or even thick rubber bushings on the saddle tubes of skids). The a/c enters a positive feedback loop and when it's pronounced enough to be noticeable, it's probably irreversible and the aircraft is going to shake itself to death.
https://www.youtube.com/results?search_query=ground+resonance
Frightening to watch, especially if you've ever been inside on of them thangs.
New to me. Fascinating.