Is this just a theoretical question?
Everyone is there with their korban Pesach and this one guy is in linewith a calf....in the Beis HaMikdash and nobody tells him before the schechting that he has the wrong animal there?
It certainly would be surprising if this had been a Ma'aseh sh'Hayah - for the reasons you give. Since it wasn't, though, we are entitled to imagine a hypothetical case - e.g. he has the animal covered with a blanket so it is not visible to others - in order to explore the concept of Ta'ah b'Dvar Mitzvah. It certainly is no stranger than other hypothetical scenarios in the Mishnah and Gemara, for instance the case of a man standing on his roof and throwing a Get to his wife below and the Get burns up before hitting the ground (Gittin 79a).
This is not a Psak Halachah