'Can't you come through properly', 'The whole thing would fracture. Two universes would collapse.','So' - In which Rose Tyler presses a button. Goes off canon at the end of Doomsday.

Trouble's just the bits in between, or so the Doctor told Jackie Tyler. Really, though, it depends on your definition of trouble. After all, what causes more problems in the long run - a thwarted attempt at a Slitheen invasion, or falling in love with a 900-year-old alien?