It doesn't happen that often though - can't think of any examples other than the Ford/Raworth switch mentioned earlier
The all time classic was an overnight News 24 shift when the absence from the studio floor of the duty presenter, one Clarence Mitchell I believe, was noticed as the top of the hour loomed.
Luckily Nicky Schiller, (formerly of this Parish and whose contributions to TVF are much missed) was producing. He has presenting experience, and jumped onto the set and covered admirably until the aforementioned Mr Mitchell was located and woken up...
Is that what led to the downfall of Clarence Mitchell at the BBC? I remember he was quite a high profile reporter during the 90s, even presenting the Six O' Clock News on occasion, but after this particular incident was never really seen again, until a few years down the line when he popped up as the McCanns spokesman.