(CNN) At 26.2 miles long, running a marathon is no small feat, with those crossing the finish line looking forward to celebrating with anything other than some extra time pounding the pavement.
But on Sunday some 18,000 runners in Belfast ended up getting just that.
In a statement, Deep RiverRock Belfast City Marathon organizing committee chairman David Seaton said that "approximately 460 additional meters were added to the officially measured course of 26.2 miles," an addition of nearly 0.3 miles to the route.
Seaton attributed the mistake to "human error, with the lead car diverting from the official route," and apologized to all competitors, saying that it would never happen again.
Many runners first noticed the disparity between what their watches said and the course length, with some posting on social media that their GPS had measured the course at up to a half mile longer than expected
The marathon, in its 38th year, had changed its route this race, with organizers boasting it as a "flatter faster marathon route" that was "perfect for a first time marathon or to seek a PB (personal best)!"
On Sunday evening, the Belfast City Marathon said that it was in the process of adjusting runners' times to reflect the correct distance.