I guess you'd see a lot of it if you have an aquarium full of pet fish. While it is not normal for fish of similar size to eat each other, they do attack each other sometimes out of territorial behavior or simply being nippy. The injured fish are more likely getting attacked and eventually die. Some fish will chew on other dead fish. More often it is the big fish eat small fish fry when some fish in the same aquarium had given birth. It is one way to keep population in control as it has been the same way in the wild.
It is usually no big deal since we as pet owners should already have advanced knowledge about what might happen if we mix up the wrong species. So it usually won't happen unless we took the unnecessary risk.
However, if a dog eats a cat, or vice versa, I'd totally freak out.
While it is not normal for fish of similar size to eat each other, they do attack each other sometimes out of territorial behavior or simply being nippy. The injured fish are more likely getting attacked and eventually die. Some fish will chew on other dead fish. More often it is the big fish eat small fish fry when some fish in the same aquarium had given birth. It is one way to keep population in control as it has been the same way in the wild. 
