At the Berlin Zoo, a woman entered the polar bear display and swam with the bears. The bruins were not amused, and nipped her several times. The headlines blared: "Polar bear attacks woman at Berlin Zoo...," --but actually it was the other way around. See the video here.
Some months later at the Bern Zoo (Switzerland), a "mentally disabled" man entered the enclosure of a bear, and was mauled. Police shot the bear, driving it into its cave so they could rescue the man. Later, both the bear and the man were recovering. More
At the San Francisco Zoo in recent but separate incidents, intruders entered both the black rhino exhibt and the grizzly bear exhibit.
Born Free USA documents incidents between humans and zoo animals. Here you can see hundreds of examples of bites, escapes, and other mishaps, where the animal is usually the loser: at the Apple Valley Zoo in Minnesota, a "9-yr-old girl is bitten when she ignores warning signs, climbs over barrier, and reaches down into enclosure in attempt to pet meerkats.... Zoo is forced to euthanize entire family of 5 meerkats to test for rabies. Meerkats test negative for rabies. (www.kare11.com)."
In the US, it's legal to shoot an intruder in your home, if you think you are in mortal danger. So why do we expect anything different from animals?