Two stories:

ONT-STL in Dec. 1991 on TWA. It was extremely foggy in STL and the plane would drop, shudder as it tried to climb and then swing from side to side. Kept doing this for the last 15 minutes until we touched down.

LHR-BOS March 2000 on British Airways. My sister and I checked in for our Virgin Atlantic flight at Gatwick only to find out it had been canceled. Virgin employees took about 30 of us and told us they would put us on a BA flight leaving from Heathrow. They bussed us over there and we board the plane. Mostly smooth for the first couple of hours. Dinner was being cleaned up when all of a sudden we hit a major drop. The man across the aisle did not have his seatbelt on and he smashed into the ceiling and landed on the armrest. He was ok, but it was a weird sight to see someone go flying like that. The severe drops continued for a few more minutes. One of the crew kept screaming into the PA system... "buckle your seat belts, remain seated for the safety of yourself and the other passengers!" This as the other flight attendants made a mad dash through the cabin to find their seats. We got through the drops and continued to BOS with very light turbulence. I happened to have my video camera on at the time and caught some of it on tape! The cabin looked like a tornado went through it!