I was following my 7 and 8-year-old brothers and we somehow scaled a wall at the zoo.  When our feet hit the ground (we hadn't looked what was on the other side of where we were climbing) we had dropped right into the habitat of a full-grown, startled, American bison.  It was a bull and he started pawing the ground and snorting at us.  Luckily, some zoo employee had seen us climb the wall, jumped in, scooped the 3 of us into his arms and quickly jumped a fence just outside the reach of the charging bull.  Wow, was that close. 

I have no idea what would have happened if he hadn't saved us.  Knowing my older brother, he'd have probably pulled me in front of him to shield himself.