(CNN) – About 30 furloughed federal employees were accidentally paid in an apparent glitch before the funds were recovered, The Washington Post reported Friday.

Friday marks three weeks of the partial government shutdown, and the first missed paychecks for the over 800,000 government workers who have been either furloughed or are working without pay.

The Post, citing a senior Interior Department official, reported that nearly three dozen employees at the Chemical Safety and Hazard Investigation Board — which assesses industrial chemical problems — as well as two other Interior Department employees had received checks from their payroll manager, Interior Business Center, after one of the employees entered the wrong payroll code, the official told the Post.

“As best we’re told, the coder for the board was a new person who did not understand how to do the coding properly,” the official told the Post.

Once the center realized the error, the board sent the employees an urgent email instructing them not to spend the funds.

“PLEASE DO NOT ACCESS THE FUNDS,” the email, viewed by the Post, read. “We are working with IBC to determine how to remedy this situation and it is best if you don’t access the funds.”

Meanwhile, the center worked to freeze the payments that had already been sent to three banks.

“There are three banks that are involved, and we’ve called the banks to trace that money and work to pull it back,” the official told the Post.

Most of the federal employees affected by the shutdown got the lack of pay they had been expecting — and the worries that came with it.

William Striffler, an air traffic controller working without pay at Newark Liberty International Airport in New Jersey, showed CNN the pay stub for $0.00 he received after working 64 hours this pay cycle.

“We’ve been working knowing that it was coming, and when it hit yesterday, it really hit,” Striffler told CNN’s Poppy Harlow.

The shutdown came just as he and his wife are expecting a baby girl, due next week.

“This should be one of the happiest moments of our lives,” he added, “and we have this hanging over our heads. And it’s really an unfortunate situation to have to worry about.”