I can think of two times when it got ugly. Once, on a flight from Denver to Boise, I was sitting in the back of the plane and the turbulence was so bad that I felt the plane was going to basically disintegrate in mid-air. I guess it didn't.

Another one, substantially more harrowing, is on a GRU-EWR flight, when the plane, a brand-new 767-400ER, lost most of the electrical system over the Amazon. No cabin lights, limited instrument functioning. The engines were still running, but could not be restarted were they to fail. We were two hours away from the nearest airport that could handle a large plane (Manaus). We headed for Manaus, and made it safely. CO sent a crew from GRU in a chartered jet, which eventually managed to fix the plane, but we ended up showing up in EWR 12 hours late (but at least we made it).