Just out of curiosity - Why do you think they made a mistake?
The vote for SNP was interpreted as a threat on England, so England voted Tory. Effectively, a vote for SNP ensured a Tory government.
Nobody could have foreseen it ahead of time, but that's what happened.
Cameron could take this all away by giving Devo Max to Scotland, basically justifying the SNP vote, but if he did he'd be doing two things - one, giving Scotland a lot of power over their own affairs, in an era of austerity sacrificing tax and spend powers. Two, it'd signal immediate electoral reform - and with the UKIP/Green surge I don't think boundary reform would cut it; there'd be loud shouts for PR.
His other option is to mandate another Scottish referendum - if he did, he'd alienate England, as the tail would be wagging the dog, after a campaign where he almost solely accused Labour of being in the same situation. Can't see it for another 10 years myself.
What I think will happen is a bunch of grand sounding but ultimately hollow concessions to Scotland, an increasingly angry but impotent SNP, and the Scottish electorate will realise independence is a pipe dream in the near future, and 2020 is best served by voting Labour and trying to ensure the Tory era ends.