There is an unspoken agreement between an author and his/her readers that the author will play fair with them and give them the clues they need. This means that, should the main character be in a pickle and the only thing that will get them out of the situation is that they have all the skills of a judo black belt, the author will not suddenly reveal that the character actually won an Olympic Bronze medal in - yes! - judo.
Similarly, they will not suddenly win the lottery/inherit a fortune in the last chapter which solves all their money problems, or the detective suddenly reveal that Mr Bloggs is the murderer, if this is the first time Mr Bloggs has made an appearance in the entire story.
Nor will a view point character know information which they hide from the reader, despite the reader being led to believe they know the view point character's thoughts. Agatha Christie notoriously did this in The Murder of Roger Ackroyd, and her readership felt both shocked and cheated.
That's not to say that an author can't lead the reader down the wrong path - in fact, half the fun of reading crime fiction is guessing (wrongly) the solution. But it has to be possible for the reader to make a correct guess. Anything else is cheating. Ditto, in romance fiction, for the heroine to suddenly declare undying love for some bloke who hasn't featured on their radar before. I've been re-reading Pride and Prejudice and one of the pleasures is watching Lizzie realise her true feelings about Darcy, long before she acknowledges them to others let alone herself.
As the presenter said in the programme, 'the clues are there.' If they're not, you're cheating the reader.
