The entry in Chambers was CALIMA until the revised 13th edition when it somehow became CALINA.
Couldn't 'flatter', be used in the comparative sense ('the flatter of two surfaces') to justify Oxymoron's use of TRUEST here? I'm not convinced it's the superlative sense of the word that's been clued.
I certainly didn't like the clue, and I thought it was a bit unfair to give a difficult clue for such an obscure unit of currency, but I'm not convinced it's an error.
Schadenfreude's built up quite a reputation for precision over the years (in this puzzle for instance, there are 15 normal entries and 15 across + 15 down thematics) I don't think he makes many mistakes.