Well, speaking from my dad's perspective..

If he got something that he didn't order, he would tell them, and they will usually bring him what he originally ordered.

If he didn't like something that was part of the meal, he wouldn't eat it. He would sometimes write an "anoymous letter" saying what he didn't like about it, hoping that they would be able to change it. Or, he wouldn't say anything. It generally depends on his mood.

For me, I would feel bad saying I didn't like the dish, so I just wouldn't eat it.