3 years ago, flying to MLE on my first 767 flight. Skipper said he had received a report of "moderate" turbulence from a Swiss aircraft a few hundred miles in front of us. We were flying over the Adriatic sea, near the coast of Albania at that time. So he put the seat belt sign on just in case and said he'll get back at the passengers with more info later. No special instructions for us so we kept doing our service (we were preparing for dinner, the 1st service of the flight).
After about 10 minutes I had to go to the loo (yes, even cabin crew have physiological needs!). I was in the process of washing my hands in the loo between doors 3 when the worst turbulence of my life started. I hit my head on the mirror and got a huge bruise

, but the worst part was I couldn't get out. I was grateful that the toilet was one of those with a vacuum pump!
I finally managed to get out but it was so bad I couldn't walk or even crawl to the jumpseat at door 3 left. The only thing I could do was to squat and hang on to the bulkhead curtain hoping that it would be strong enough to support me!! The cabin crew at the back had food flying everywhere; as soon as he could the first officer made a pa to take our seats immediately (I thought it was a joke!

) by which time we were all sitting wherever we found space, some crew sitting on each other's laps secured by the belt.
When after about 10 minutes the situation got better (it was a LONG 10 minutes) the cabin was a mess. a third of the 300+ passengers had been sick or were feeling ill, some with panic attacks, the food was all over the rear and front galley, and a few of us had bruises in many places (we counted them the following day on the beach

)
When things returned to normal the captain informed us crew that we had dropped almost 3,000ft at one point, and that he had to descent from 36,000 to 15,000 feet to find some "clearer" air. The plane though had behaved very well, despite a drop of almost 3,000ft the autopilot had not disconnected.
Now, for somebody who started flying because she was terrified of turbulence I felt soooooo proud of myself as I was calm and could reassure the pax around me, however I wished tele-transportation had been invented already during that week in the maldives!!!
I have had many other occasions of moderate turbulence since then (approaches in GOA and LGW-DUB flights are the worst) but none as bad as that thank god!!!