CORRECTION: The concerns Ofqual expressed in 2014 about the OCR examination board related to results from 2014 and were not a prediction of marking mistakes in 2015, as an earlier version of this article suggested. Ofqual has also asked us to clarify that it did not forecast "a record number of appeals". The article further referred to exam boards' "guesstimating" of grades. Ofqual has pointed out that estimating grades is necessary in a small number of cases and that there is no evidence that these estimates are significantly inaccurate, or that the practice could lead to a "slew of mega-mistakes" as the article originally suggested. We apologise for the contrary impressions given. The article has been amended.