Serving coagulated dips
Cold queso is unforgivable.
Refusing minor substitutions
Chefs have specific visions for their dishes, but when ego stands in the way of a customer's experience, they've gone too far. Of course there are exceptions for dishes that wouldn't function without specific ingredients, but it's a mistake not to do everything possible to please the folks paying the bills.
Letting soups sit
No matter how "of the day" it is, soup is often an afterthought. Once it's simmering, the only real legwork is ladling it into a bowl, which some chefs can still manage to mess up. "You cannot keep a soup on the hot line for a long time as it can form a coating on top, especially gumbo," says chef Austin Kirzner from Red Fish Grill in New Orleans. "Soup needs to be ladled and served a la minute."