Hello and welcome to Pet talk! A couple of quick questions about your cat..
Does she seem to get a little aggressive during petting sessions? Does she ever become agitated during petting or is it always when you're finished?

There are many causes when it comes to cats and biting/scratching behavior. A lot of times it is a form of aggression, and again...there are different kinds of aggression. The way your Mercedez is acting says to me that it is a overstimulated and excitement form of aggression. This type happens when a cat gets overstimulated (possibly too much petting, or touching a sensitive spot without knowing it). Cats that like to be handled can sometimes get to a threshold that means...ok, that's enough..you can stop now. Or, on the other hand, she may be saying...what! Are you crazy? Don't stop petting me yet! Depending on when she gets aggressive, you can safely assess what it is that is causing the behavior. If she does bite or scratch you in an attempt to keep you petting her, say OUCH! Really loud and then ignore her. This should over time tell her that it's not appropriate to bite or scratch, and that it will not give her her intended results. Lavish her with praise when she doesn't exhibit the aggression and give treats. Hopefully this will be helpful!