"I know a cat."
But it actually isn't. In Accusative of masculine nouns, it matters whether the noun is animate or inanimate. This is the only situation where it matters.
If it's animate, then Accusative is identical to Genitive. "Widzę kota" = "I see a cat" (Accusative), "Potrzebuję kota" = "I need a cat" (Genitive).
If it's inanimate, then Accusative is identical to Nominative. "Widzę stół" = "I see a table" (Accusative), "To jest stół" = "This is a table" (Nominative).