This would be a disaster so we do absolutely everything we can to avoid that happening. The part of work relevant to this question is ‘Quality Assurance’. So once we’ve designed something we have several other more experienced people ‘check’ the design (calculations, drawings, specification) to make sure there are no mistakes. Once the structure is being built there more checks – before concrete is poured I would check the steel reinforcement cage has been built correctly, and someone else will check the concrete has been mixed correctly. There will even be people checking the structure is being built in the correct location – it needs to be mm perfect!
On one bridge I designed, the contractor (company who builds the bridge), did some quality checks on the foundations. This showed that there were holes in their concrete so then I had to check my design to see if the structure was strong enough even with the ‘weak’ foundations. In the end I found that these foundations were ok if they were partly repaired, but on a similar structure which also had this problem, a whole new set of foundations had to be built. This caused a delay and additional cost to the project.
On existing structures when we are inspecting them we will be looking out for ‘critical defects’ such as cracks in metal. As a crack can propagate (grow) very quickly, it would be reasonable to stop traffic over/under the structure if one was found until it was fixed or made safe. This is another way we protect people – however it is very rare to find cracks in metal structures!