An unfortunate error meant maths students were set a question that was impossible to answer in an AS-level exam.
Just under 6,800 teenagers took the paper - set by the OCR exam body - last Thursday.
OCR has apologised, saying it will make sure candidates are not disadvantaged by the mistake.
But some students writing on social networking sites have been calling for the test to be re-run.
The error was in an exam paper taken in 335 schools and other exam centres in England, Wales and Northern Ireland.
Rest of article which includes the question: http://www.bbc.co.uk/news/education-13627415