[Keith]

Has it ever happened before? Someone skipping an event?

I think it might have happened in the original series after a contestant sustained an injury in the previous round, but hadn't been medically ruled out of continuing. I'd assume they'd only allow for a maximum of one round to be forfeited.

There was certainly instances in the original series where during an episode a contestant couldn't continue, so the standby contestant took over mid-episode. I can't remember if there were any instances of a contestant being unable to take part in the quarter/semi-finals, though back then there was usually 15+ episodes per series (after the first series) so more potential to use runners up.

The producers probably would rather they forfeited the 5th round than be unable to take part in the eliminator. At that point it wouldn't seem right to use the standby contestant, nor have no eliminator or have just one contestant starting the eliminator. I think there were a few instances in the original series of a contestant being unable to complete the eliminator though.