Peter Davison tells a tale on one of the WHO commentaries (it might be on TERMINUS, if it was Lisa Goddard in SKIPPY), but he says that (Lisa?) told him that what used to happen on Skippy was that they had loads of kangaroos for the filming and one time, during a scene Skippy had to jump into the river for some reason, but the poor thing sank straight to the bottom. Nobody bothered rescuing it, leaving it to drown and the director simply said: "Break out another Skippy!"