This might be the 1957 accidental incident, when a youngster fell into the water and it's mother jumped in to save it, only to scramble out on the other side of the moat.I believe a chimp, Gloria, did though.
The other I'm aware of involved the legendary Compo, he escaped through one of the ventilators and was spotted by leader Solomon, who then deliberately crossed the water to tackle him only for Compo to disappear across the fields after a brief fracas.
That incident prompted the installation of a hot wire in the moat. After Solomon investigated a couple of times, to the best of my knowledge it successfully dissuaded them from trying any further escapes.