When Katie Davison opened her AS exam results this summer she was confident they would match her teachers' predictions of all As.
Last year almost 6,000 A level grades were changed.
She was devastated and confused to discover she had got two As and an E grade.
The results severely restricted her choice of university and she appealed. She has just found out her grade has been changed from an E to an A.
While she is delighted with the outcome, the whole appeal process has left her deeply upset and has fuelled teachers' worries that the new online system of marking exam papers may be to blame for mistakes.
Katie said: "I was very disheartened when I got my history results. I had no interest in continuing with the subject and no enthusiasm for applying for university.
"I was very happy to get the correct result but I have friends who are now wondering if they have the right ones."
Her teachers at Tolworth Girls' School in Surrey said they had unprecedented numbers of pupils wanting their exam results checked this year.
They want to know if that is linked to the rise in "online" marking, where pupils' exam papers are scanned and emailed to examiners for grading.
Katie's history teacher, Julie Waite said: "I have not seen in my experience - and I have been teaching many, many years - a candidate whose mark has doubled in this way, particularly without any explanation like this."
She says since the deadline of 20 September has now passed for pupils to question their results, many are now wondering if their exam grades were as widely wrong as Katie's.
Ms Waite said: "It's deeply concerning for other pupils and undermines their morale and belief in their own marks.
"Because it took five weeks to get the results back it's now too late to question any other pupils' marks."
Edexcel is one of the exam boards which is championing online marking.
It says it has total confidence in the system and aims to mark every paper online next year.
While online marking does streamline the process, some examiners say it's difficult to mark some papers accurately, particularly where answers are in essay form.
While there is no hard evidence of a direct link between incorrect results and online marking, teaching unions say they are seriously concerned about the system.
The Qualifications and Curriculum Authority, which oversees exams and marking, says concerns should be raised with them directly for them to be investigated so that students get the results they deserve.
Figures for last year show almost 6,000 A-level grades were changed.
While this is a small proportion of exams sat, John Bangs from the National Union of Teachers fears a rush to mark papers online may cause more problems.
He said: "There have been enough questions raised for a real in-depth, open and transparent look at the nature of online marking so that youngsters can be confident that their papers are being marked accurately."
While Katie can now relax knowing she has the right grades, many other pupils may now be worried that theirs are incorrect ones.
Her mum, Sue Davison said: "Katie is the tip of the iceberg. This issue is too important to get wrong.
"We are playing with young peoples' lives and really examination boards need to sort themselves out to make sure this doesn't happen again."