Here in Southern California we have an amusment park called Knotts Berry Farm. They have a train ride that circles the park and you just relax and enjoy the view. During this ride a couple of bandits (old west) emerge from the back of the train with masks and guns and demand your valuables. I've noticed over the years that this scares little kids (and some adults that didn't know it was going to happen). After the kids see that it was an act they are ok and laugh about it. This has been going on since I was a little kid.
I think if a representative from Knotts made an announcement prior to the train leaving that this stunt was going to happen, it might ruin the effect.