Around two years ago, when mathematics researcher Jean Ecalle submitted a paper to Acta Mathematica Vietnamica, he saw that he had the option of making the paper open access. So he checked a box on the submission form — which included a mention of the fees that he apparently missed — and didn’t think anything of it.
The paper “Eupolars and their Bialternality Grid” appeared online in October, 2015.
So Ecalle was quite surprised when, sometime later, he received an email from a representative of the publisher saying he owed 2,640 Euros. He responded in January 2016, guessing what the fees might stem from: