Pupils sitting a GCSE exam were baffled when they found what appeared to be a business studies question in a biology paper.
A question in the AQA biology exam reportedly asked pupils to define an "independent company", prompting streams of children to complain about the paper on social media.
There were more than 100,000 tweets about the exam within hours of the paper being taken, but the exam board has denied there was any mistake.
The candidates' dismay comes after 25,000 people signed a petition claming that the Scottish N5 maths exam – equivalent to GCSE – was "unusually" hard and calling for the pass mark to be lowered.
Jack Green, 16, a pupil at South Hunsley School in North Ferriby, East Yorkshire, said pupils were "worried" about their grades as they thought the exam was very different to the syllabus they had been taught.
As well as the "independent company" question, Jack said the exam also contained a question about 15-year-olds' favourite alcoholic drink.
"It was like, 'What is a teenage boy's favourite drink?' – I don't have a clue what that had to do with biology," he said.
Jack rang the board to complain about the paper.
"They were making out like it was a mistake, the guy on the phone said that something went wrong. It's stuff that we've never been taught and that's not the teachers' fault, it's the exam board's fault because they didn't tell the teachers to teach us it.
"I'm hoping that they'll either let people through with their target grade, which is something they've done before, or recall the papers and send out a new one.
"It's my first GCSE exam that I've done and it's like, 'Are all GCSE exams going to be stuff that I haven't been taught?'"
'Students were struggling'
He said that many other pupils in the exam hall appeared to be struggling.
"A lot of people you could see were stressing out. Every time I looked up to look at the clock there was someone there tapping their pen on the desk, agitated."
"Everyone's a bit worried because this biology exam has such a high weighting for the total grades, so it's really affecting what the overall grade will be."
Jack, a singer, is going on to college to study music, media and business. He said: "Hopefully with business studies I might learn what an independent company is!"
An AQA spokeswoman said that the "independent company" question was one of several on the subject of drug trials, and that the underage drinking question was in a section on alcohol abuse.
The exam board said that both questions were included on the syllabus.
"Exams aren't meant to be easy and students are obviously going to tweet about that, but there was nothing wrong with this paper," the spokeswoman said.
"We wish everyone the best of luck with the rest of their exams."
When asked by the Press Association to define an "independent company", an AQA spokeswoman said students had studied how independent organisations are said to be unbiased.
AQA said the syllabus referred to scientific evidence being scrutinised for any potential allegiance or bias, and that an independent company was one with "no allegiance or bias towards the drug company referred to [in the question]".
Ex-teacher Pete Langley, who founded the study support website Get Revising, said there were a "record number" of posts about the AQA biology exam on The Student Room – an online forum.
"It sounds like students have had a really tough exam and are feeling disappointed after all the hard work they've put into preparing for it," he said.