I politely tell the server and get the problem corrected. Quality restaurants will get the situation resolved. It might be well to also tell the Manager in case this is an on going problem.
depends - say you hate cheese and your meal comes out covered init then be polite and send it back if it just a few peas on the side that you dislike just push them aside. when you start making a fuss some chefs get funny and you wouldnt want them splitting in it
I hate cheese and if they give me something with cheese on then i tell them im allergic to it and if i eat it then my throat swells up and they are fine with it!