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.
The question carried eight marks out of 72 being awarded for the paper.
One candidate Thomas Fay, who contacted the BBC News website said he had been distressed to find a question that appeared “impossible”.
“This threw me in the exam and many people found this to cause much added stress in the exam,” he added.
“Many people are worried that the mistake made by the examining board will severely affect the mark and grade they achieve in the paper. For many this was a final exam and will most likely influence final grades and university admission.”
Dozens of other students have messaged the BBC News website to voice their anger and fears about their grades.
Aron De Vos, 17, from St Albans, said: “I spent a good 15 minutes trying to answer that question. I was getting very frustrated about why I couldn’t get the answer.
“I want to retake that exam. I can’t believe how much time was wasted on a question where we were only able to get zero marks.”
OCR has said it deeply regrets the “unfortunate error” and says it has a range of procedures in place to ensure candidates are not disadvantaged.
A spokeswoman said: “We very much regret that there was a mistake… and that our quality assurance procedures failed to identify this error.
“Because we have been alerted to this so early, we are able to take this error into account when marking the paper. We will also take it into account when setting the grade boundaries. We have sent a letter to all schools and colleges explaining in more detail what we shall do.
“We do apologise again that this has happened.”
The exam body says it is not going to discount the question from the marking, because that might disadvantage candidates who spent a lot of time trying to answer it.
Students will be awarded points for their attempts to work out the question and measures are also in place which are designed to recognise that other candidates may have discovered the error quickly, OCR says.
OCR released full details of the error – on paper “Decision Mathematics 1″ – as follows:
- The question as printed asked candidates to verify the shortest route, for two given conditions, giving values of 32.4 + 2x km and 34.2 + x km. These values should have been 34.3 + 2x km and 36.1 + x km respectively. The error was not to have included twice the journey between A and B (0.9 km) and the journey between F and G (1.0 km) in the values given.