Aww this happens all the time - engineers are human, they make little mistakes when adding their sums up...
When the tram lines were being built in Dublin it was discovered that the gauges of the tracks were different. That was an expensive mistake, not as bad as a tunnel though :)
We have a tunnel to take trucks away from Dublin, but the supertrucks can not fit in it.
That was done on purpose. The city policy makers decided that they didn't want supertrucks in the city at all.