I love J K Galbraith (the second best economists of the twentieth century and a massive influence on my early economic thinking) for saying it.
But the introduction is first rate too, so I make no apologies for reproducing this:
In 1958, Harvard economist John Kenneth Galbraith was looking for a term to describe certain ideas that were commonly held, intellectually accessible and yet fundamentally flawed. To define such widely spread misconceptions he wrote: “I shall refer to these ideas henceforth as the conventional wisdom.”
What's the conventional wisdom now?:
a) That there are bond vigilantes;
b) That cutting government spending will result in an immediate uplift in private sector activity
c) That there is significant and abnormal level of inefficiency in the state sector.
None of those things is true of course.
But they are the conventional wisdom.