While this doesn't include entering an enclosure, per se, I believe my grandfather deserves a mention. For context, both incidents take place at the Fort Worth Zoo. When I was about seven I had a birthday party at the zoo, and during an educational program where they brought animals into the room, my gramps reached over, attempting to pet one of the zoo's resident African Penguins which then bit his finger. Recently he also tried to pet a Giraffe I was feeding, I told him not to and he could get bit and/or get in trouble, but he continued whilst STANDING DIRECTLY ACROSS FROM THE KEEPER AND SECURITY GUARD. They gave him a warning and I began laughing as he began going on about how he could pet them when he was younger.