By Dominic Casciani
The "secret" documents clutched by Mr Quick were clearly on show
It is every detective's worst nightmare. A massive investigation compromised by a slip that a graduate fresh off police parade ground should have spotted.
Assistant Commissioner Bob Quick's resignation has implications for the UK's counter-terrorism operations because it has forced a sudden rearrangement of Scotland Yard's top desks.
But the bigger issue is what damage the mistake has done to an operation which aimed to thwart a possible al-Qaeda plot, after officers launched it before they were ready to do so.
Counter-terrorism operations are among the most complex and dangerous.
If things go wrong, then the public are put at risk.
If things go very badly wrong, such as with Jean Charles de Menezes on 22 July 2005, then innocent people die.
The events of the last 24 hours arguably put public safety at risk in two different ways: people could have been harmed - and an eventual prosecution could be compromised.
When detectives have gathered evidence against a serious suspect, they do not pile into squad cars like Gene Hunt in Life on Mars.
If they have the time, police chiefs draw up a careful plan to minimise the risk to the public by arresting the suspects in as discreet a fashion as possible.
This usually means a "dawn raid" with a huge team of specialist officers sealing off streets.
They move in during the early hours and, when the target properties are surrounded, gain entry at exactly the same time to secure their suspects.
The whole business can take just minutes - and can go largely unnoticed if the officers don't need to smash their way in.
The threat is contained, the public are unaware of the danger - and nobody is anywhere near the scene who should not be.
Contrast that to what happened on Thursday. Arrests were made in broad daylight by armed officers.
One man was filmed on a cameraphone being arrested at gunpoint outside the main library at Liverpool John Moores University.
The library's public address system was used to warn students to keep back for their own safety.
Two other men were arrested at a Homebase DIY superstore in Clitheroe.
This was far from ideal - particularly if the police had been forced to shoot.
Andy Hull is a terrorism expert at the Institute of Public Policy Research.
Arrest: In broad daylight at Liverpool John Moores university
He previously worked for the Metropolitan Police Authority where he scrutinised major terrorism operations.
He says that senior officers exhaustively work through the implications of an arrest operation, checking and double-checking their duties to the public.
"The police meticulously plan for these kinds of arrests," said Mr Hull.
"But they also know they must plan for external factors beyond their control - the 'known unknowns' to quote Donald Rumsfeld.
"Senior officers have to think through back-up plans if things go wrong."
A classic example was 22 July 2005.
Cressida Dick was the gold commander in charge of the hunt for the failed suicide bombers.
Her notes, revealed at Jean Charles de Menezes' inquest, show the steps she took to ensure that her officers on the ground knew what they had to do.
During the Met's subsequent trial, the jury said they wanted no blame to be placed on her shoulders because of the professionalism she had shown in extreme circumstances.
Evidence for trial
But ultimately arrests should only happen when there is enough damning evidence for trial.
Community reassurance: Key part of terror operations planning
It is no good to prosecute someone just on the basis of ambiguous conversations recorded by a bug.
So what matters is the material that convinces a jury that the defendants were not just talking the talk, but walking the walk.
In 2004, police arrested a group of men involved in a plot to build a homemade bomb.
The swoop only came after a long surveillance operation that yielded hard evidence - not least a storage room full of an explosive ingredient.
In contrast, security services around the world have all had bad experiences in major operations where they have moved in too quickly and later found they did not have all the evidence required for a conviction.
It is the spectre of this scenario - knowing that you have the right men but have failed to get evidence - that haunts counter-terror police chiefs.
A botched arrest operation can have wider implications.
Early arrests, particularly in complex organised crime operations across many countries, can risk tipping off other suspects that the police are on to them.
Evidence can be destroyed, false passports obtained and disappearances organised.
In this age of instant communications, it is a risk sometimes not worth taking.
And in the case of al-Qaeda associated plots, police know they need to reassure Muslim communities and prepare a careful media strategy.
The Bob Quick papers photographed outside Downing Street include references to the media plan.
Security expert Professor Michael Clarke of Kings College London told the BBC that he doubts this particular operation has been seriously compromised, other than becoming a public relations disaster.
We do not know a great deal about the men whose arrests were triggered by Bob Quick's mistake - and they have not yet been charged.
And in practical terms, the picture of the Quick document does not give away a lot of detail. But the type of detail it does reveal could be enough for conspirators to work out that the police are on to them.
Dame Pauline Neville-Jones, the Conservatives' security spokeswoman, said there was no guarantee the police would have all they need because they had to move in early.
"The question arises as to the quality of the evidence," she said.
"One doesn't know that the operation has been prejudiced, but obviously if the police are not able to act at a moment of their choosing, it reduces their chance of success."