When I was about four or five years old the skytram ride in Busch Gardens Tampa broke down and we had to evacuate the emergency route. It took us through some keeper-only paths with only a thin wooden fence separating us from herds of livestock. The guide was giving us some kind of safety talk, but naturally, being four, I wasn't interested. I climbed up onto the wooden fence, and spotting a herd of ankole cattle on the opposite side of the paddock, decided to call them over by repeatedly yelling "moo!" One by one they lifted their heads to look at this little boy standing on their fence wearing red shorts on a windy day (I know, myth, but funny detail), then they decided to charge en masse. I only just managed to jump down before around 10 huge cows all smashed against the fence at me. Good thing it was strong enough!
More recently I was observing the Asiatic lion in the indoor exhibit in Edinburgh when it decided to charge me as well. Thank God for barriers!