Exam boards have been given a public dressing-down over the quality of their question papers by the qualifications watchdog, following a series of embarrassing mistakes.
Teachers fear pupils' university places could have been jeopardised by questions that were impossible to answer.
Three such mistakes in three separate AS-level papers issued by each of England's main exam boards have been publicised in the last week.
And exams regulator Ofqual told The TES that it was also aware of "two other similar cases currently being investigated", but said it was not prepared to give details of them.
In a letter, released to the press, the watchdog warns the boards that such errors can disrupt exam candidates and cause them anxiety. Ofqual also calls on them to carry out further checks on papers being taken during the rest of the summer.
"I am aware that where such errors do appear in question papers, awarding organisations are able to implement steps to limit the impact on the students' marks and grades," Ofqual chief executive Glenys Stacey writes.
"However, incidents of question errors can clearly disrupt students when taking an examination and cause anxiety as to how this will impact on their final results.
"In some cases these errors have made it impossible for the students taking these examinations to generate the correct answer for the question concerned."
The OCR board admitted it made an "unfortunate error" in printing an unanswerable question worth eight of the 72 marks available on an AS-level maths paper.
One teacher said some of their pupils had got "stuck in a rut", spending up to a third of the 90-minute exam on the impossible question and then failing to complete the paper.
An AQA business studies AS-level paper sat last month contained another unanswerable question worth three out of 80 marks.
The latest case to emerge involves a multiple-choice question on an Edexcel biology AS-level paper, which offered a selection of wrong answers without a correct one.
An Edexcel spokesperson said: "The marks for this paper will be adjusted during marking and standardising to ensure no candidate is disadvantaged."