A poorly (or un-) supervised child at Disney's California Adventure. Mrs.k2002 and I were there on Monday and were in line for Toy Story Midway Mania. After a nearly hour-long wait, we had nearly reached the boarding area when the whole ride shut down. As they were letting us out (and giving us return passes), one of the cast members explained what had happened.
The boarding area(s) for the ride are about two feet above the track level. Here's a picture of the regular boarding area.
There's really no risk of someone falling onto the track because of how the gates work when you board. The second area where they transfer people from wheelchairs and with other mobility issues is more open, because they need the room to maneuver people into and out of the vehicles. Apparently, some child either jumped or fell down to the track level. There were cast members there, but they were helping with the boarding, not minding the children. When someone ends up near the track that have to hit the emergency stop, which shuts all of the power off, immediately. It takes them more than an hour to restart the whole ride, since maintenance has to come and inspect the track for damage. They also have to go back and escort the people who were trapped on the ride to an exit. Then do test runs to make sure everything is ok. All because someone couldn't keep control over their child.