After David Cameron's announcement this morning of Conservative plans for a tax on banks, Will Straw wrote a post on his Left Foot Forward blog suggesting that a senior member of the shadow cabinet had publicly opposed the policy.
Headlining the post "Calamity Grayling opposes Cameron's unilateral bank tax", it was accompanied by a YouTube clip of the shadow home secretary, purporting to back up his story.
Alas, in the clip, filmed at a public debate in his Epsom and Ewell constituency, Mr Grayling had been asked for his views on a transactions tax, which is something completely different.
Commenters on the Left Foot Forward blog made this clear in some numbers – to the point that comments on the post have now been closed.
Fraser Nelson has eloquently told the tale in its entirety here.