"We're aware of an error in today's GCSE English Literature paper. We apologise and can assure everyone that no student will be disadvantaged," The Oxford, Cambridge and RSA Examinations (OCR) tweeted.
Geoff Barton, general secretary of the Association of School and College Leaders, said when candidates see errors in a paper it can undermine their confidence.
He added: "Candidates have every right to expect that awarding bodies complete a full check on exam papers to ensure that they don't experience such problems.
"Similarly, schools and colleges have to pay thousands of pounds a year to examination boards and are entitled to better quality assurance than this.
"This appears to be a serious error and it will have caused stress and concern to candidates."
However, some students reacting to the exam on Twitter said that they had wasted time on the question, while teachers also spoke out.
One GCSE pupil, @alimount50 said: "I'm absolutely fuming.... had to learn 15 poems and they couldn't even get a household right."
Another, @LuciaBowler tweeted: "So even @OCREnglish can't get the Romeo and Juliet families right how do they expect me to get it right?"
OCR said about 14,000 students out of around 700,000 took the exam, but it is not clear how many tackled the question, which asked students: "How does Shakespeare present the ways in which Tybalt's hatred of the Capulets influences the outcome of the play? Refer to this extract from Act 1 Scene 5 and elsewhere in the play."
In a statement, the exam board said, "We apologise and will put things right when the exam is marked and graded so no student need worry about being disadvantaged. We are investigating as a matter of urgency how this got through our assurance processes."
Russell Hobby, general secretary of school leaders' union NAHT said: "After two years of study it's not acceptable for students to face such a basic error in their exam papers.
"OCR are rightly apologetic but just exactly how they propose to mark this particular paper will leave many students, their schools and their parents with an anxious wait for their results," Hobby said.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)