Revealing mistake: In the opening scene the Doctor and Donna are speaking over the phone. The purple phone Donna is holding shows the slot where a cable should be plugged in, but there is none. (00:00:25)
Suggested correction: This shouldn't be considered a mistake. While it's true in real life this phone has a slot for a cable, they're on an alien planet and this is unknown technology. In-universe there's no way of knowing what that slot is for, and there's no reason to think the phone needs to be plugged into anything.