Ironically, the key skills test was for communication, at level 3. The exam was taken by 40,000 people.
There was widespread confusion when candidates reached a question about animal rights which referred to two images - one pro-hunting and one anti-hunting. Questions asked students to refer to images "one" and "two", but the images were not labelled with a number.
"It was a proof-reading error on our part," said a spokesman for the authority. "We make no excuses and this error should not have happened, but we have made sure no students suffer as a result."
The offending question was worth 10 per cent of the test marks but the QCA says the mistake will be taken into account.
The papers are prepared by a group of award bodies on behalf of the QCA but it says its own proofreading, or possibly a data corruption caused by electronic transfer of materials, was to blame.
Mistakes are inevitable in an examinations system which deals with 18 million papers a year, says the QCA, but the Association of Colleges insists standards need to be improved.
"The system cannot continue like this," said Judith Norrington, the AOC's director of curriculum and quality. "We cannot have the future of young people and the reputation of the public examination system regularly called into question by these continuing errors. There are too many weak links in the chain."
The AOC has asked the QCA and the awarding bodies involved, including Edexcel, to work together to prevent a repeat of the problem.
AOC officials say they have had numerous complaints about mistakes in exam materials.
"The awarding system is a very careful one," said Alan Dass, of the QCA.
"Chief examiners have been immediately alerted and it is their job to make a fair decision about the question, looking over the paper as a whole and I have no doubt they will do that.
"What is important is to swiftly put things right and make sure no student is damaged."