In October, Gu Gu bit a teen intruder. And in 2007, he bit a drunken tourist who jumped into his pen and tried to hug him. The tourist retaliated by biting the bear in the back.
The moral of the story? When your 5-year-old drops a toy into the panda pen, and the panda has attacked twice before, don't jump the five-foot barrier to get it!