Let's set the scene.
You are dining with your companion at a restaurant you are quite familiar with. You know what's good and, more importantly, you know what to avoid, in this case the salmon.
In an adjacent table is another couple who are within earshot and you overhear that they are considering the salmon.
What do you do? Do you warn them? Or say nothing?
Or be passive-aggressive about the situation and say to your own dining companion, in a rather loud voice so that adjacent diners can't help but overhear, how awful the salmon is at this particular restaurant.
What do you do?