Part of a sliproad beside the Taney crossroads in Dundrum is about 200 millimetres too high to allow heavy goods vehicles to pass under the Luas bridge nearing completion there.
The gaffe is far from being a disaster in construction terms, however, according to project resident engineer Mr Paddy Barry.
"It means we have to reduce the road levels over an area of five by three metres. It will take two days at the most to put right but it won't hold up the project or cause any delays to traffic."
Dundrum, he said, was "a happy place" despite the massive construction projects involving the €44 million bypass and the €8 million, 180-metre cable stay bridge. "Everyone on the site here is happy, the shop-keepers are happy and the Luas bridge is looking good."
Mr Barry dismissed as "totally without foundation" reports that the sliproad was a major problem for the bridge project.
His colleague, Mr Tony Dempsey, at the consulting engineering firm, Roughan and Donovan, which designed the bridge, was equally emphatic about the scale of the problem.
Trucks and buses were going underneath the bridge without hindrance, he said. It just meant a small part of the road would have to be regraded. "When we were designing the bridge we made sure there was enough tolerance, so that we could open or close it, if necessary, during construction, if there was any problem."
He too was dismissive of reports that it represented a real setback for the Luas project, with repercussions for the Dundrum bypass. The problem stemmed from a kerb being installed "at the wrong level" at the construction stage of the sliproad by Dún Laoghaire-Rathdown council. The consensus was that it was a relatively minor problem and would not cost the taxpayer a penny extra.
Mr Philip Brown, chief engineer for Grahams, the building and civil engineering contractors, said it was being built "according to plan".