When the first finishers crossed the line at the San Francisco Half Marathon on Sunday, the reaction was immediate.

"Everyone said, 'Oh my God, that's an amazing time,' " said San Francisco Marathon spokesperson Lyz Luke.

No wonder.

It turns out that six or seven runners at the front of the race accidentally cut roughly a mile off the 13.1-mile course, leaving all of them disqualified. Worse yet, the majority were "elite" runners, who had chosen that race to try to post a fast time.

"We are incredibly apologetic about it," Luke said.

The event was one of two half marathons sponsored by the San Francisco Marathon, which was also held Sunday. Apparently, it was the overlap of the races that caused the confusion.

It happened at Stow Lake in Golden Gate Park. Runners in the second half marathon saw runners racing around the lake and followed them, not realizing they were from the other race. When they noticed their mistake, Luke says they "climbed over some barriers," inadvertently cutting a mile off the route.

"We're thinking maybe we should get some better signage out there for next year," Luke said. "Maybe a few more course monitors."

Duh.