In the normal course of events, the pressure of time would have forced the detectives to make a decision without benefit of legal advice on how critical the contradiction was.
Instead, they went upstairs to the station's criminal justice unit and put their concerns to their resident legal guru, Roger Thatcher, Principal Crown Prosecutor.
Within an hour, they had their answer - they should not attach undue weight to the anomaly, allowing them to charge the adult involved. This case illustrates the potential value of a year-long project that has put senior Crown Prosecution Service lawyers inside police stations around the country, including London, Durham, Northamptonshire and Humberside.
"This is where the project is breaking new ground," said Mr Thatcher, who divides his time between Shoreditch and Hackney police stations. "I was able to consider the crime report and discuss the material from the video interview with the girl and give my advice very quickly.
"In the vast majority of cases, defendants are on bail while the police work on the case. It can be weeks before the CPS lawyers get the papers and, if they then require further evidence, a lot of time can have elapsed. Under this scheme, officers can stick their head round the door and any problems can be considered there and then."
The idea of posting prosecutors into police stations was announced last year by the Attorney General, Sir Nicholas Lyell, as part of a package of measures to counter public and police resentment over CPS decisions to drop or reduce charges.
Mr Thatcher, 48, has been a criminal lawyer, first as a solicitor and then as a barrister, for all his 24-year legal career. He acknowledged the frustration police officers and the public could feel when there appeared to be reasonable evidence of a person's guilt, but it was difficult to prove.
"Even if you have a fair understanding of how the criminal justice system works, it can be frustrating because you want to see justice done," Mr Thatcher said. "But the difficulty is that the system has grown up over many years to protect the defendant rather than to find the truth or deliver justice."
The CPS has found itself the butt of officers' frustrations. However, the hope is that if one of its lawyers is on hand to explain a decision, the reasons will be better understood and accepted than if it comes via a memo.
Since the CPS was set up in 1986, it has had to make its mark as the independent prosecuting authority. So was there a danger that its independence could be compromised by closer liaison with the police?
Mr Thatcher, who joined the CPS in 1987, said he believes it is now strong enough to work more closely with police at the early stages of cases without risking its objectivity.
"Lawyers are trained to carry out a particular role and it is their professionalism that creates the independence, not geographical distance," he said.
"Lawyers have a duty to give independent advice in whatever field they work - it is not about telling the client what they want to hear or giving advice that is not founded in reality and common sense."
Chief Inspector John Grundy, in charge of the criminal justice unit at Shoreditch and Hackney, dismissed any likelihood of collusion. "I suppose it is human nature that if an officer has invested a lot of time and effort in a case, he could try to persuade the CPS lawyer that there is a viable case.
"But I do not see Roger being swayed simply because he is here rather than in a CPS office.
"He is proving to be a first-class ambassador for the CPS. Inevitably, with two organisations like the CPS and police, there is a degree of ignorance about each other's roles that can lead to misunderstandings and misconceptions. Having a member of the CPS here, putting himself on offer for advice, helps to dispel that."
Mr Thatcher also gives advice on general issues, such as entrapment, the effect of the new caution that limits defendants' right to silence during interviews, and any other legal points raised by the officers.
Identification often poses tricky questions. One query put to Richard Parsell, Senior Crown Prosecutor, while he was duty lawyer at a Darlington pilot scheme was whether a suspect with a tattoo on his face could be put on an ID parade. What would be the court's view if the police stuck a plaster on him and put identical ones an everyone else in the line-up?
"My advice was go ahead with the ID parade ,but do not draw attention to the tattoo by covering it up," he said.
Advice given under the pilot schemes is recorded on forms that are signed by the lawyer and officer, with a copy being added to any file later submitted to the CPS. The aim is to ensure quicker decisions, a reduction in wasted effort and changed charges, and higher-quality police files.
The Director of Public Prosecutions, Barbara Mills QC, writing in Police Review, emphasised the need to get cases right: "Endeavouring to get it right at the last minute frustrates the interests of justice, causes delay and runs the risk of compromising strong cases and prolonging genuinely weak ones."
Hilary Bradfield, Branch Crown Prosecutor in Northampton, hand-picked a team of four experienced lawyers who work on a rota basis at the town's two main police stations.
"What we are trying to do is help to ensure the charges are right first time so everybody knows where they stand. Instead of the police deciding, for instance, to charge someone with a Section 20 wounding when we would prosecute it as a Section 47 ABH at court - and everybody is left saying why couldn't this have been sorted out at an earlier stage?
"In cases involving conspiracy, for example, we do not want to be reading loads and loads of statements obtained unnecessarily because they will never prove the case. If we can give advice early on and suggest areas to concentrate on, then it saves the police time and resources and saves us time later on."
Mark Haslam, secretary of the London Criminal Courts Solicitors Association, welcomed the project, with the proviso that the CPS lawyers were sufficiently senior to make decisions on the spot.
"If it means that in even a few cases people are spared the agony of long waits to hear if they are going to face charges, it will be a step in the right direction," he said.