after bad. Huge sums have already been sunk into the project, runs the argument, and all that - plus political credibility - will be lost unless it continues.
The fallacy weaves an astonishingly powerful spell over ministers. Only after spending £500m on a computer system upgrade and three years watching it fail did Labour ministers finally do the right thing and throttle the congenitally useless Child Support Agency.
Governments of every political hue fall prey to it. Since 1993, American taxpayers have been bankrolling the International Space Station, which has produced nothing but cost overruns, and whose final price-tag is likely to top $100bn.
About the only good news in the Commons committee's report about the NHS IT upgrade is that, so far, only around £2bn has been spent. That alone is a hefty sum - enough to pay the wage-bill of 10,000 extra nurses for a decade. But unless ministers can cure themselves of the sunk cost fallacy, it will be just a downpayment on a debacle of historic proportions.