A real-life case of what's allowed once becomes carte blanche:
My uncle and aunt recently moved to my parents' town. They lived with my parents for a couple of weeks while they were waiting to close on their house. Uncle and Aunt have two small dogs that are Aunt's babies. Obviously, while they were living at my parents' house, the dogs had to be there, but it about drove my mother crazy. She's never wanted pets, and the dogs peed on some of Mom's carpets. Aunt and Uncle didn't say anything about it, but Mom could smell it. After Uncle and Aunt moved out, Mom had all the carpets professionally cleaned.
When Aunt and Uncle showed up for Thanksgiving dinner, they had the dogs with them. I thought my mother was going to have a heart attack when one of the dogs ran into the kitchen and made a beeline for my daughter, who was playing on the floor. (I admit, I wasn't too thrilled about it either -- I don't mind dogs around my baby, especially because we have a dog, but I would have liked a little warning that the dog was there and loose so that I could keep an eye on them.)
My mom didn't say anything about it to them, but she was really steamed. They didn't ask or even tell her that they wanted to bring the dogs. Plus, they showed up two hours later than they said they would, hadn't made the dishes they said they'd make, and whipped out all the ingredients to make the dishes from scratch, delaying dinner by almost two hours, but that's a whole 'nother story....