Even the best behaved and most watched children can and do wander away from time to time. Then again, that's generally in situations where the parents let their guard down. I remember the first time we took our little kids to the zoo - I was paranoid and kept an extra vigilant eye on the kids. Ditto Niagara Falls - 3 foot railing, then it's over the side you go. I still remember how tense I was, keeping an eye on the boys. (Actually, I think it's at least 42" at Niagara Falls, but in some areas, no fence at all, just a short rock wall.)
Also, Hello Ornery!