Thirteen A-level students missed out on university places last summer after marking errors by one of England's biggest exam boards left them with the wrong grades.
An inquiry into mistakes made by the Assessment and Qualifications Alliance (AQA) laid the blame on failures with the board's online marking system and the awarding body was criticised for its delay in reporting the problem to exams regulators.
Isabel Nisbet, chief executive of the exams regulator Ofqual, which led the inquiry, said the failings by AQA were "very disappointing."
Errors occurred when a failure with AQA's online system meant that not all of students' material was marked, the inquiry found.
In total, some 3,353 GCSE and A-level pupils were given the wrong marks. Of these, 622 students were given the wrong grades, including 146 A-level students.
Ofqual said that the university admissions service, Ucas, had confirmed that while the majority of A-level students were unaffected, in 11 cases students were offered places at their second choice university, or placed through clearing, rather than their first choice. A further two students failed to find places at all.
Ms Nisbet said: "The failings by AQA identified by this inquiry are very disappointing, especially as they led to some candidates missing out on their first choice of university or course.
"Factors that contributed to the marking error included limited piloting of the new on-screen marking system, a lack of effective risk assessments and deficiencies in the role and training of examiners on the new system."
In this case, Ofqual said it could have "reasonably expected" AQA to have notified it on September 17, when the failures were first discovered.
In a letter to Ms Nisbet on Wednesday, AQA chief executive Andrew Hall said AQA "deeply regrets this failure and the impact it had on students".