Currently working on a public sector construction project. The way these things work (which could form a PhD thesis in stupidity in itself) is that Design Team A produces an options report, scores the options with the client and selects the preferred option. The project gets approval at government department level, the client gets planning permission. The whole thing then gets re-tendered and Design Team B (us) pop up to complete the detail design before handing it on to a contractor. It's exactly the kind of long-winded Kafka-esque bureaucratic idiocy the government forces on the public sector in the name of efficient oversight but which would never be tolerated in the private sector.
On this occasion the preferred option just does not work because assumptions which were made about constructability are not valid. When we were first appointed we said, 'This doesn't look right, you need to think about revisiting the options selection process.' Client says, 'No, you work with the approved scheme, carry on'. To the point where, now, we're looking at adding maybe £4m cost to a £7-8m project, and even then it will be so complex a lot of contractors will either refuse to touch it or will give such a stupid price it can never go ahead. We've told them what the fix is.
My understanding is that the fundamental reason we cannot change tack is because it would be a bit embarrassing for the client body to have to go back to the government department and admit they dropped a bollock and they need to start again. Apparently it's less troublesome to stand by a 150% overspend.