Rollbacks can happen, although they are rare. Small rollbacks are slightly more frequent and they aren't usually noticed by the people on the line.
The rollback videos were shot in Winter Park on a lift that WP was replacing, so the Colorado Tramboard staged an absolute worst case scenario on the lift. They disabled everything in order to make that happen. A simple press of the E-brake stop everything, because they enable the bullwheel brake and they sawed off that brake in order to do this test. Fun stuff though!
I was a liftie at CB for 3 years and the lifts are extremely safe, and though no completely idiot proof...it takes a lot for a lift to rollback. That said there are numerous other ways for things to go wrong on a lift....