Quite a few years ago, during the Tour of Ireland, didn't Sean Kelly come to a halt/fall-off going a hill near the finish, & have to be pushed up by his team-mates, as it was too steep to set off normally??
Now, if that horse of a man couldn't restart..................
Worst climb that I know, that has caused chaos in the 'Tour of Britain', & especially on a damp/wet day (when it's a complete nightmare) is one that's called 'Shibden Wall' (officially Lee Lane)
I'm not sure if that's a local name, or just one that was made-up by the race presenters??
it's in the Shibden Valley, at Halifax (OS 104; SE 095 273), & climbs past the Shbden Mill Inn.
I know that my own attempts to ride it have failed in the wet.
Then, on normal roads, 'The Strines' in South Yorkshire (Mortimer Road) are the hardest I've ridden regularly