But I've been wondering afterwards what her mistake was, precisely. As leader of a party with one MP, she doesn't need to use the meaningless language of Westminster discourse. There was no need to put a figure on the number of homes they would build.
The spurious number of 500,000 new homes was mentioned, and it is worth saying that - even during the Macmillan and Wilson years of jerry-built high rise flats, the UK never managed more than 400,000 a year.
Conventional political wisdom says that you have to express these things in terms of numbers and costs or nobody believes you, but is that really the case? The figure of 500,000 a year is too round a number to be believable, too imprecise to be serious. And either the basic policy work had not been done, or Natalie Bennett had forgotten the details.
I was irresistably reminded of the fatal moment when Charles Kennedy revealed a less than complete mastery of the details of the Lib Dem proposals for a local income tax during the 2005 general election. His wife had just given birth, so perhaps it was understandable - but it was a critical moment too. I expect this will be a critical one for the Greens.
But before Lib Dems get too holier-than-thou about it, it is worth remembering that they are making precisely the opposite mistake to the Greens.
The Greens have not worked through the practicalities of their proposals in sufficient detail. They are not focused enough on immediate policy, but they have sharpened their ideology and everyone knows what they are for.
The Lib Dems are hugely exercised with the short-term strategies of getting policy details through Whitehall and the coalition. Their whole attention is on making things happen, but have forgotten - hopefully temporarily - that they exist for a purpose beyond the moderation of Conservative and Labour excess.
If they forget sometimes what they are actually crusading for - the fundamental purpose of the party and its ideology - the Greens never do. So don't let's be smug about Natalie Bennett's embarrassment. She is at least beginning to think about the practicalities of radical policy-making.
I want the Lib Dems to do well this year as much, if not more, than anybody. Nobody could accuse them of stinting on the policy front. There will be powerful green policies in the Lib Dem manifesto. But the more I can persuade the party to provide that crusading edge - to remember what their long-term purpose is - the more I can improve their chances.
Starting perhaps with recommending my own attempt at radical practicalities in my new book (written with Tony Greenham) on the practicalities of ultra-local economic regeneration.
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