It’s one thing to forget to change your clock from daylight saving time. It’s quite another to mistakenly close the entire Metro system an hour early.

But the second-busiest subway system in the country did just that over the weekend. Riders were stranded early Sunday morning, and lots of questions remain unanswered about how Metro managed to lose track of time.

The transit agency had announced plans to stay open an hour longer so that, even with the clocks being turned back an hour, the system would remain open until 3 a.m., its normal weekend closing time. But what Metro is calling “human error” botched the transit agency’s plans. Riders who showed up at stations expecting trains to run until 3 a.m. were locked out and had no way home.

Metro spokesman Dan Stessel would not explain how the error happened or who was responsible but said employees could be disciplined.

“A mistake was made. It shouldn’t have happened, and we apologize to everyone who was inconvenienced by that mistake,” he said.

Riders reported being stranded on U Street and finding locked entrances at Dupont Circle, two busy night-life areas in the District.

Many people who are out late in those areas over the weekend come into the District from Maryland and Virginia, said Elizabeth Szatkowski, a manager at Marvin, the popular bistro on 14th Street NW. For those people, going home is more than a quick cab ride across the city, she said.

“I can’t even imagine how stranded some people were,” she said. “It’s really inconsiderate.”

And she pointed out that an unexpectedly early closure is also a problem for restaurant employees who have to turn around and spend the money they just earned on cab fare.

“It’s definitely a point of concern,” she said. “I mean, everybody can’t afford to take taxis.”

Stessel said they had received about a dozen complaints as of Tuesday morning. Refunds will be decided on a case-by-case basis, he said.

The premature closure was no easy undertaking.

Station managers had to close the 84 stops that were open (Rosslyn and Arlington Cemetery were already closed for maintenance). Station managers had to lock up all of the gates, operators had to make final stops and other Metro employees all had to head home, despite it being an hour earlier than trains were scheduled to stop running.

“We’re going to take steps to make sure it doesn’t happen again,” Stessel said.

The transit agency will pay closer attention to the station’s closure the next time the clocks change, he said.

Metro’s daylight saving practice has typically been to close an hour early in the spring and stay open an hour late in the fall.