Category assignments aren't always fully unambiguous; some items either belong in different categories depending on perspective (e.g. cats are both pets and carnivores) and some items have enough similarities that although there is a natural categorization (dolphins are mammals), there are other categories that are tempting (e.g. dolphins "are fish").

You avoid category mistakes by paying attention, and making sure that when you reason about category instead of actual thing, that the item you've placed in that category really does have the key properties you're using in the logic.

Beyond that, it's hard to say much at this level of generality.