I remember going to LAX about 16 years ago in on of our BA 747-400's. I was sat in D zone (behind the wing, but not the most rearward cabin). In the vicinity of Greenland having just climbed to FL350 we encountered some unforecast turbulence which ended up as severe after about 10 minutes.
I reassured a pax travelling next to me that all would be fine, but the sight of the engine pylons oscillating and the wing tip travelling over a range of about 20 feet whilst twisting from root to tip was quite alarming!
Internally, anything not strapped down was bouncing off the roof panels (incl pax), and the sound of breaking glass resonated from the secured galleys.
About two minutes into this, the speedbrakes were extended and the aircraft turned off track and descended back to FL310, where peace returned.
After the event, as a pilot I offered my assistance to the cabin crew in the galley, several of who were very stressed and tearful.
Afterwards, I spoke with the flight crew (had a few beers in the Huntley that evening), and they relayed that in about 60 seconds they had 3 stall warnings and 3 overspeeds - the instruments were totally unreadable. The skipper confirmed it was the worst turbulence he had experienced in 30 years of longhaul flying.
I have certainly not experienced anything like it since, and as such am very wary of describing turbulence as being 'severe', when 99+% of the time it rarely exceeds 'moderate'.