The loss of an A-level exam paper in Amsterdam has led to calls for a free re-sit from UK students who sat the replacement paper.
The exam board Edexcel sent out replacement papers after learning about the missing copies - but two schools in the UK and two overseas mistakenly gave students the original ones.
Students say the new paper was too hard and are calling for a re-sit for all.
Edexcel says there is no evidence that the lost papers were leaked.
But it will monitor the results and take action if necessary, it says.
The paper is C3 Unit 6665 mathematics and the exam board says the old paper was taken by 60 students, while the replacement was taken by 34,000 in the UK.
Exam papers went missing when they were being delivered to a school in Amsterdam in mid-May, according to Edexcel.
Students are complaining on Facebook and Twitter, saying it is not fair that they will be graded with others who have taken a different version of the exam.
Others are saying the paper was extremely hard and had questions that were not on the syllabus.
One who contacted the BBC said: "The original paper has been replaced by one which had topics which were completely off syllabus.
"This is an important exam for a lot of people as a lot of people have university offers based on grades that are going to be achieved."
The exam board has no plans to arrange a re-sit.
Another student told the BBC she feared she would not now get the top grades she needed to take up the offer of a place at Cambridge.
On Twitter, student Shamma Khan said the real issue was that the paper was too hard: "Each question was the extreme of difficulty. I had to check if I was doing the right paper.
"They build you up to practise, study, revise then break you down by changing patterns."
One maths teacher told the BBC News website the paper was the hardest he had seen since the 1980s, but others have said that although it was a challenging paper, it had all been on the syllabus.
Paul Caira is the head of maths at a school in east London.
"We had 15 of our very able pupils in tears in the examination room," he said.
"While the questions were technically within the syllabus, the style was so out of line with earlier papers that pupils justifiably felt a huge sense of being wronged, which I share."
But maths coach and author Colin Beveridge, said in a blog: "There were a few unusual questions, and I rate it as a tough paper that required some thought beyond mechanically applying formulas - which is one of the things C3 is meant to test.
"At the same time, there were several questions that should have been almost automatic for a candidate with a realistic hope of doing well in the exam."
Exam board Edexcel said the replacement paper had been designed to the same standard as the original one but that if examiners reported that it had been too challenging for candidates, this would be taken in to account in grading.
A spokesman said the exam board had checked and all schools had received the replacement papers.
"There is always a small risk that a small number of candidates will accidentally sit the original paper, as has happened in this case.
"At this stage we believe that students at two UK and two overseas centres sat the original paper. We are continuing to monitor these numbers and would like to reassure students that all exam scripts will be marked, and we will carry out statistical analysis to ensure that all students are fairly rewarded for the performance on either paper."
Examiners will watch out for near-perfect scores to try to find out whether the original paper had been circulated.