Voters wait to cast ballots at Coventry Hills School in northeast Calgary on Monday night. (Meghan Grant/CBC)

The city's returning officer is looking at how to prevent future ballot shortages at polling stations.

Barbara Clifford said on Monday night that any station that had a 22 per cent turnout by 4:30 p.m. was sent an extra 800 ballots.

The city ended up couriering out extra ballots to 90 polling stations.

At one — Prince of Wales Elementary School, in Ward 14 — there was a mix-up and the ballots didn't get delivered, Clifford said.

"Upon arriving at the Prince of Wales Elementary School voting station, the courier was unable to make the delivery because the school was locked," a city news release explained. "An election worker was stationed inside the school to wait for the courier but did not see or hear the courier."

That left about 100 voters still waiting to cast their ballots at 8 p.m., when polls closed.

Some left when they heard the ballots had run out

Others were able to vote for aldermanic and trustee candidates, but not in the mayoral race.

Brand Inlow, the city's manager of regulatory affairs and special projects, called it "an extremely regrettable situation," and one that they make great effort to avoid.

"Generally speaking, we distribute … 60 to 70 per cent of ballots. In other words, you'd need that degree of turnout before you ran out of ballots, which is historically a very high margin of safety," Inlow said Tuesday.

Inlow said that during the "unprecedented" voter turnout, it became quite difficult to communicate with the polling stations.

He also pointed out that the danger of sending out more ballots than needed is that the returning officer has to account for all ballots, used or not, and a surfeit would only lengthen the already time-consuming process of counting them.