that some of their pupils had got "stuck in a rut" and spent up to a third of the 90-minute exam on the impossible question and then failed to complete the paper.
Another teacher pointed out that some pupils would have just moved on to the next question.
"The big problem is that a large number of students have university places that depend on this exam and need to achieve particular grades, often A or A* with no margin for error," a teacher wrote.
OCR has written to schools and colleges apologising for the mistake and explaining its response.
"We are very sorry that our quality assurance procedures failed to identify this error and recognise that this would have had an adverse effect on candidates' performance," the letter reads.
A spokesman for exams watchdog Ofqual said: "OCR has promptly reported the incident to Ofqual and is investigating. It will report back to us and we will then take action as necessary."
An OCR spokesperson said: "We very much regret that there was a mistake in Question 6(ii) of this paper. We would like to assure teachers, parents and students that we have several measures in place to ensure that candidates are not unfairly disadvantaged as a result of this unfortunate error.
"Because we have been alerted to this so early, we are able to take this error into account when marking the paper. We will also take it into account when setting the grade boundaries.
"To help us understand how this occurred and to minimise the chance of such an error happening again, we will be undertaking a thorough review of our quality assurance procedures."
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Original headline: `Unanswerable' maths question threatens university places