No one was more surprised than Alan Johnson when he was appointed shadow Chancellor just over three months ago. He even attempted to turn his inexperience into a joke. Asked what his first move would be in the job, he replied: "Pick up a primer – 'economics for beginners'."
But the quip – and the suspicion he only got the job because it would have been too risky for Ed Miliband to give it to Ed Balls or Yvette Cooper – haunted him during his brief tenure.
Only last week David Cameron taunted Mr Miliband: "We have ended up with a shadow Chancellor who cannot count and a Labour leader who does not count."
The problem for Mr Johnson was that he handed plenty of ammunition to his Tory critics.
He warned that this month's rise in the rate of VAT would make food more expensive, when in fact almost all foodstuffs are free of VAT.
He was caught out in a television interview when he was asked to give the rate of employers' national insurance contributions. After failing to avoid the question, he answered that it was going up from 20 per cent to 21 per cent.
Told that the rate is, in fact, 12.8 per cent, he admitted: "There's still a lot to learn for you in terms of this job."
Equally damaging, he claimed the last Labour government (of which he was a senior member) planned to eradicate the national deficit by "about 2015-16." A few hours later he confessed he had got his numbers mixed up and "probably meant 2016-17".
Alongside the gaffes, the shadow Chancellor also created problems for Mr Miliband by making little secret of their policy differences.
Mr Johnson, who backed the elder Miliband brother for the top job, contradicted his new leader over how long the 50p top rate of tax should remain, the merits of a graduate tax to fund universities and whether to reduce the influence of the unions over Labour.
His interventions sparked accusations that he was pursuing a Blairite agenda, forcing him to insist he agreed with his leader after all.