1. When there's a foreign object in your food. Like a hair or a bug. Or if you're really unlucky, a Band-Aid.
2. When there's an ingredient you're allergic to. Especially if you asked them to leave it out, or it wasn't listed on the menu.
3. When your food is undercooked or overcooked. Especially if it's way off. Like you order a steak medium-rare, and they bring it to you well done.
4. When something that's supposed to be hot comes out cold. It's usually because your server is busy and forgot about it. The same applies for stuff that's supposed to be cold. Like if they leave your salad under a heat lamp too long, ask for a new one.
5. When it's not what you ordered. Obviously if they bring you the wrong food, send it back. Or if they have to remake it and you just want to leave, ask if they'll discount your appetizer, or anything else you got.