And it was this: the conflation and confusion of 'freedom of expression' with 'freedom of the press'.
It is particularly misleading in the context of the debate around regulation. Regulating the press will undoubtedly have an impact on the 'freedom of the press'.
But regulating the press is frequently described as a move that would have a damaging effect on 'freedom of expression': the more regulation of the press you have, the more 'freedom of expression' will suffer.
What the phone hacking scandal has demonstrated is that quite the opposite can be true: a lack of regulation has meant 'freedom of expression' has suffered. Phone hacking epitomises the way in which 'freedom of expression' can and has been undermined by the 'freedom of the press'.
I cannot see how 'freedom of expression' has been advanced by the risk that a 'free press' will hack into individuals' private expressions of their views in voicemails and publish them to the world.
And this is merely one aspect which made an impact because it was also illegal.
Every time the press publishes a headline which has no resemblance to the story beneath or sensationalises a story or rips a quote out of all context or strips people's Facebook profiles for information or decides it would be in the 'public interest' to silence anonymous bloggers or simply makes up a story, the 'free press' assaults 'freedom of expression'.
Essentially then, the 'free press' damages 'freedom of expression' in two arenas: perhaps most shockingly in the ever-complicating 'private' realm by putting freely expressed private statements into the public domain, but also in the ever-complicating 'public' realm by distorting public announcements.
The latter is one of the reasons why the PR industry in the UK has grown so substantially. Whether you are a politician or representing a company or increasingly an 'ordinary person', it has become necessary to pay people for expert advice on what you are able to freely express in public.
And if the boundaries between the 'public' and the 'private' realms continue to blur, how long will it be before each of us needs our own personal PR adviser to help us work out what we are safe to freely express in our increasingly public private lives?
I believe a 'free press' is vital for democracy. But the time when a 'free press' should have started using that freedom responsibly to stand up for 'freedom for expression' rather than abusing it to turn a profit has already passed.
We need a more effective system of regulation which protects 'freedom of expression' from a 'free press'. And at the same time, we need to ensure that a newly regulated 'free press' is also able to protect 'freedom of expression'.
Not least because 'freedom of expression' faces much greater threats, both now and in the future, from companies and governments around the world.