My old mum had £1 EW on Foinavon in the National. It won at 100-1. When I went to collect her winnings the bookie asked: 'How the **** did you pick that?'
I remember that race Foinavon was so slow and so far behind the others that when the entire rest of the field fell at the last fence (due to a loose horse running straight across as they were about to jump the fence) Foinavon was able to pick a way through the chaos and just get to the post in time , even then he was so slow he was very nearly caught at the post.
Possibly the only time a horse has won a race by being the slowest in the field.