Exam board AQA has apologised for including an unanswerable question in its A-level Chemistry paper.
Pupils sitting the exam this afternoon were told to leave out one of the questions in the paper.
Schools were contacted about the mistake before today's exam to ensure students could be told, AQA has said.
But some pupils on social media claimed that they were only notified in the middle of the exam – and some say they were not told at all:
Our invigilators didn't tell us about the mistake so we just corrected it and did it anyway. Fuming #aqachemistry— zo (@_zelizabeth) June 13, 2017
When you only find out you were meant to cross out question 10.3 because of twitter #AQAchemistry— Yasmin (@YasminB_1) June 13, 2017
how can our exams officer stop the exam midway to say they've had a call from aqa about 10.3, bonus marks soon come #aqachemistry— victoria (@XIIII_) June 13, 2017
Pupils will be awarded the two marks from the question, which has had to be discounted.
An AQA spokesperson said: "This shouldn't have happened and we are sorry it did.
"Each of our exam papers goes through a lot of checks, but unfortunately errors can very occasionally slip through.
"When they do, we always make sure that students don't lose out. But one error is still one too many, so we are carrying out a detailed investigation".
The mistake came on the same day Tes revealed that exam board OCR had made an error in its Biology A-level paper.