Three examples from me, two of which were on United.
May 24, 1996. SMF to ORD on a 727. Seat 3A. Beautiful day, only some whispy clouds here and there. But before landing, Capt. Sky comes on, tell the F/As to finish service and sit down. "Hold on, it's gonna be a bumpy ride." Holy crap, no kidding!

We jumped and bumped all over the sky. Some would say better than the wildest roller coaster. Normally, I have no problem with turbulence, but that day it really got to me. Now mind you, I am not subtle when I barf. I can't control it.....but I have a deep gutteral growl that starts in my toes and all the muscles work from there, going up, to empty my stomach. There is nothing diplomatic when I need to ralph. I'm gonna do it and everyone knows it. Funny thing was, about 20 mins after landing, when we get to RCC and try to re-book our missed connection, two guys walk up and stand in line behind us. One says to the other "Yeah, that was a rough flight. And I can't believe that guy in First Class, you could have heard him back in Sacramento."
Second, August 2001. Small prop plane, flying over the Masai Mara from a camp to Nairobi. Most people thought they were just some moderate warm-air up-drafts. But I had been on Larimum (anti-malarial) for 3 weeks. If any of you have read about Larium, you know the wierd things it does to you, psychotic dreams, anxiety attacks. Soon, the flight got a bit worse, and I think everyone was looking around, just a bit nervous. ["Are we going to end up in a small, 2-paragraph story back in the states:
12 Tourists Die in Plane Crash in Kenya"] I had a major panic attack due to the larium combined with the turbulence. If I had been at an exit room, I could have jumped, just to get out of that plane. Never had that happen before or since. (So, this story only half- related to true turbulence.)
October 2005, NRT-LAX, 747, Seat 2A. Meal service was about half thru, when we hit some speed bumps in the air lanes. It gets progressively worse. After meal is done, and we settle down, it's a major bump, bump, bump. Capt comes on and has F/As sit. Although we never dropped any distance, so nothing went airborne, several passengers had glasses spill onto them or drop to the floor. We had seatbelt sign on for over 6 hours. Capt came on about every 30 mins for an update. Even the F/As, who on this route have high seniority (boy, that's the nice was of saying it!), said it was the worst flight they had ever been on.