Why were hundreds of vaccine appointments canceled? Too many people cut in line.

To prove eligibility for a COVID-19 vaccine, many Americans have brought to their appointment a pay stub from work, or a doctor’s note verifying a legitimate health risk.

But what happens when no one really checks? Because, for reasons including long lines, some vaccine centers aren’t.

On Tuesday night in Pasadena, California, hundreds of people were told that their vaccine appointments had been canceled because too many people got appointments who shouldn’t have.

Lisa Derderian, a spokeswoman for the city, spoke with ABC News’ daily podcast “Start Here” about what happened.

“We were going to have our largest mass vaccination clinic -- up to 1,500 people,” she explained. “We have this down to a T as far as how much stuff we need to support it. Our contract, nursing volunteers -- we have a lot that goes on behind the scenes, very methodical to move this forward in a very smooth process.”

Several days ago, the city emailed out a link to those who were eligible.

“The restaurant workers, grocery workers, day care teachers -- so it's a very large group,” she said. “In sending out the link a few days ago, it's very clear in a big red font: ‘Do not share.’ And it's very specific as to who qualifies.”

The city’s goal was to maintain “a manageable group to where we can make phone calls and verify that they either live or work in the city of Pasadena, because that's also mandated,” said Derderian, who soon got an email from a reporter asking whether the link was valid because she knew she wasn’t yet eligible.

“We went through our health department, and we can see the back end of our system to where people register,” she said. “Within the first hour, we had hundreds of people, none of them qualified. They were not in the current tier based on their occupation listed. And a majority of them were in like soap opera production companies … entertainment production news.”

Many didn’t even live in Pasadena, she said.

“Within a few hours, it was up to over 900 people, more than half of what we had allotted,” she added. “So, as a result, we had to cancel.”

It was a tough decision, but the only decision, Derderian said.

“There were just way too many people -- there's no way we can spend the time and staff resources making phone calls,” she said. “A lot of people were made aware of it, that they did not qualify and were not eligible and should not have gone in and done it.”

Derderian said many of those vaccines should’ve gone to seniors.

“They're hard-to-reach, a vulnerable community in many cases,” she said. “They don't trust when you call because they're told, ‘Don't give information,’ because there's so many scams out there.”

Many vaccinations that should have happened but didn’t have since been rescheduled.

“Fortunately, today, we're able to [reschedule at] a private vaccination clinic,” she said. “We spent hours and a lot of staff time calling and rescheduling so that they could get the vaccine in a timely manner, because when they are here 28 days later, you can get the second dose and it could be 29 days. People get very anxious and nervous.”

People not yet eligible, or refusing to prove their eligibility, still have shown up and demanded vaccines, with situations escalating to where police had to be called.

Some people “just start yelling and screaming, thinking that's going to help them, which it definitely does not in this situation. So it's something that we're aware of. We will check and verify because we will abide by the rules.”

Derderian also said vaccine workers were seeing “doctored up” paperwork and that what some people have tried to pass off is “amazing.”

“We've seen almost everything,” she said. “Sometimes it's hard to question, you know, it's not legit, but is it worth arguing?”

Guidelines are set by the state and rolled out in phases -- and the next one, Derderian said, could prove even more challenging because it includes people with preexisting conditions.

“There's numerous ones that are listed,” she said. “We haven't been [told] how that would work. ... That's going to be a challenging process as far as how to verify.”

“We appreciate those that follow the guidelines and the rules and wait their turn. President Biden has said by the end of May, whoever wants to get vaccinated will. And I think we will have more of an influx of vaccines coming our way in the near future. In the past several months, [we’re only] really as good as the amount of vaccines that we get. And we haven’t been getting very much.”

This report was featured in the March 11 episode of “Start Here,” ABC News’ daily news podcast.

"Start Here" offers a straightforward look at the day's top stories in 20 minutes. Listen for free every weekday on Apple Podcasts, Google Podcasts, Spotify, the ABC News app or wherever you get your podcasts.