The famous 'hippo killing a zebra' incident happened at the Basle zoo, where the mix had worked for many years. Apparently the zebra frequently fed from the open mouth of the hippo (how weird!), and then one day the hippo was startled by something and clamped down, with tragic results. There were other species in the mixed enclosure as well--antelopes and wading birds.
At Busch Gardens, the "riverbank" that encloses hippos was originally inhabited by a troop of baboons, which I believe could freely access the hippo portion of the exhibit. In recent years the baboons were replaced by ring tailed lemurs, a strange (stupid) mix.