There are actually two slip-ups here.

In the episode 'Lisa the Vegetarian' Apu states that he is a vegetarian to Lisa in the Kwiki-Mart. Later on when they're up on the roof of the Kwiki-Mart with Paul and Linda McCartney, Lisa asks why people cant be happy with eating 'Fruits, vegetables, grains and cheese'. Apu then says 'Cheese? Oh no' and Lisa asks if he eats cheese and he says 'I do not eat any food that comes from an animal'. Correct me if Im wrong, but isnt that called a Vegan?

The other slip up is at Homer's BBQ. You can clearly see Apu in a couple of shots of this scene. If hes a vegetarian/vegan, then why is he at a BBQ, that's full of meat products?