England's exams watchdog is investigating a total of six mistakes in exams taken by tens of thousands of students.
The latest are in a geography AS-level paper and a business studies GCSE.
Ofqual has said the series of errors in this year's public exams are "disappointing and unacceptable".
It has ordered urgent extra checks by exam bodies amid complaints from students and teachers.
The National Union of Students is calling for an urgent inquiry.
More than 90,000 teenagers took the papers concerned.
The six exam papers being investigated by Ofqual cover A-level, AS-level and GCSEs taken in England, Wales and Northern Ireland.
A seventh has been identified by the Northern Ireland exam board and regulator.
Ofqual says five are AS-levels (which can count towards an A-level) and one is a GCSE.
On Thursday afternoon, this was confirmed as being the CCEA business studies GCSE, taken by 3,400 students in Northern Ireland and 500 in England.
In the geography paper, a diagram showing the flow of a river was mislabelled in the first question.
A head of geography in the south of England told the BBC News website: "The question was impossible to answer correctly in my opinion.
"They should cut that question from the paper. Being the first question, it really threw some of the students.
"I contacted the exam board but have had no response to the questions I asked."
The AQA exam body has confirmed the mistake and apologised for it, saying all students would be given full marks for the question. It says the question carried four marks out of 50.
It also said there had been a problem with a diagram on its computing AS-level paper on 7 June, saying: "An arrow in a diagram was shorter than it should have been, which may have confused some candidates".
A spokeswoman said: "We are very sorry for these mistakes and will ensure that no students will be disadvantaged as a result."
The errors affect tens of thousands of students, many of whom will be relying on the results for their university applications.
Student Edwin Li Ping was affected by a mistake in his biology AS-level, set by the Edexcel exam body.
"For the first 15 minutes I looked at that question when I should have spent one minute on it," he told the BBC.
He wants to study medicine and says he "would really like to get that A".
The National Union of Students has written to the Education Secretary Michael Gove to call for an independent inquiry.
It says students whose university places are affected, or who are forced to take re-sits as a result of the errors, should be given compensation.
Shane Chowen, from the NUS, said the errors had become a "deeply concerning" trend.
"Those students who have been confronted with unanswerable questions may have had their confidence knocked and their performance in the rest of the exam affected," he said.
The head of Ofqual has written to all the exam boards on behalf of the exam regulators in England, Wales and Northern Ireland asking them to carry out extra checks on exam papers.
Ofqual's chief executive Glenys Stacey told the exam bodies to make sure measures were in place to deal with any errors that occurred.
"While the vast majority of question papers taken so far have been free from error, there have been a number of question papers that have included errors," she said.
"We take instances like this very seriously. I am calling on awarding organisations to take steps now to protect students from further disruption and anxiety."
The watchdog says in general the exam bodies have a good track record.
The exam boards involved in the other confirmed mistakes say measures are in place which will ensure candidates do not suffer.
The errors on the other AS-level papers involved:
- A maths question which was impossible to answer because not enough information was given (OCR)
- A business studies question which did not give enough information (AQA)
- A multiple-choice biology question which gave several possible answers but not the correct one (Edexcel)
The marks at stake varied from just one out of a total of 80 in the biology exam paper, to the maths question, which was worth 11% of the total marks on the paper.
AS-levels are qualifications in their own right but are also taken by teenagers as the first stage of their A-levels.
Some of those who messaged the BBC News website said they were worried the mistakes could cost them a place at university.
'Thousands let down'
Natalie Edwards, from Stevenage, told the BBC: "The exam was hard enough without having mistakes on questions that are designed for people to be able to pick up the easier marks. I couldn't work out why I wasn't getting any of the answers in the paper, which caused me to get stressed."
Sarah Streiber, from Wales, said she took both the business and maths papers in which errors appeared, and was hoping to go to university before increased tuition fees are introduced in 2012.
"And now I have potentially lost that chance! Thank you very much to the examiners who have let thousands of us down," she said.
The exam bodies have released a statement as a group, saying nearly all of the tens of thousands of examination questions published each year are error-free.
They add that extra checks are being carried out.
Director of the Joint Council for Qualifications Jim Sinclair said: "Awarding bodies are aware that a small number of questions in this year's exam papers have contained errors and understand the distress this may have caused students.
"Students and parents should be assured that no one will be disadvantaged as a result of these mistakes. Examiners marking the papers are aware of the incidents and will make careful adjustments so that all students receive the marks they deserve."
Ofqual has powers to name exam bodies which make errors, and to direct them to take certain actions to rectify the situation.
Its ultimate sanction is to withdraw recognition from an awarding body - effectively removing its powers to set and mark public exams.
Northern Ireland's Council for the Curriculum, Examinations & Assesment (CCEA) said there was also a problem with its further maths AS-level paper, which was taken by 122 students in Northern Ireland.
In this case, it said: "An error had been identified as part of CCEA's quality procedures and the paper was sent to the printers for correction. During this process a new error was introduced by the printer.
"This related to one part of the last question on the paper and was worth four marks out of a total of 75 marks."