In 2009, President Bill Clinton thanked Ben Sangari for the work he did developing public school science curricula. The following year Sangari was on stage at a Clinton Foundation event again receiving applause for his achievements in education.

Four years later, that all came to an end.

That’s because in 2014, Sangari was locked up in an upstate New York detention center, about to be deported.

Sangari, a citizen of the United Kingdom, is one of thousands of people who get deported every year for staying in the United States for longer than permitted—in some cases, like his, only for a few days.

And as Sangari watched Donald Trump’s immigration speech last week, from his new home in Canada, he found the mogul’s comments about the lack of visa enforcement utterly perplexing.

“If you overstay your visa, you’re deported,” Sangari said. “This is what they do already. It’s done. They’re doing it by the millions.”

As Sangari found out firsthand, U.S. immigration law makes life really difficult for people who visit the country on visas and then stay longer than allowed.

Overstaying a visa is a civil offense. But in Trump’s America, it would be criminal. Criminalizing visa overstayers, which his plan proposes, would mean the millions of tourists, entrepreneurs, and businesspeople who visit the U.S. every year would all be seen as potential criminals. And every visa would be a countdown to a prison sentence.

Trump has promised, both in his immigration town hall with Sean Hannity and at his major immigration speech last week, to prioritize deportations of people here who have overstayed their visas. But the GOP nominee hasn’t been clear about how exactly he would do so. Immigration attorneys say he would likely take one of two routes:

First, Trump could direct immigration officials to actively hunt down, lock up, and deport the millions of people in the U.S. who have stayed here longer than their visas allowed. Immigration agents would camp out in front of their houses or try to ambush them as they go to work. That would result in a massive number of deportations, and it would be a big step toward turning the U.S. into a police state.

“It’s a very intensive manhunt,” said immigration attorney Bryan Johnson. “It’s a needle in a haystack.”

Four million needles, in fact, in 4 million haystacks.

Alternatively, Trump could opt out of conducting millions of nationwide manhunts. Instead, he could just require that those who overstay their visas and then get flagged by police—for instance, if they drive over the speed limit, get pulled over, and then give their passports to police officers—will get deported. And guess what? That already happens.

Just ask Sangari.

Sangari had a distinguished career in education before his immigration trouble in the U.S. He is a British citizen of Iranian heritage, and he founded and then sold a company that helped develop science, technology, engineering, and medicine curricula for Brazilian schools. The program drew attention from American educators, so he came to the U.S. to look into developing it for schools in New York. And he didn’t just get attention from teachers. In 2009, Bill Clinton wrote him a letter praising his work, as the Buffalo News reported.

In 2010, Arianna Huffington highlighted Sangari’s work onstage at the Clinton Global Initiative’s annual meeting. There, the foundation praised his plan to bring his curriculum to six underserved American schools. Huffington called Sangari onstage. Then Bill Clinton shook his hand, and the audience applauded.

Sangari sold his company in 2012 and then traveled through Africa and Europe for about two years with his long-term partner, Arlita McNamee. In early 2014, he came to the U.S. with her. She’s a U.S. citizen, and he entered legally through the visa waiver program, which allows foreign nationals from participating countries—including the U.K.—to visit the U.S. for up to 90 days. Millions of foreign nationals use the program to visit the U.S. every year; Department of Homeland Security statistics show that in 2014, for instance, about 22 million people used it to come to the U.S. (PDF). Sangari was one of them. His papers expired in August 2014, but he said he decided to overstay because he and McNamee, who were then engaged, were gathering the paperwork necessary to apply for a fiancé visa and eventually get permanent residence. That September, he was pulled over for driving 50 miles per hour in a 40-mph zone. He gave the officer his paperwork, the officer realized he had overstayed his visa, and that was that.

“The side of the road was my last memory of Buffalo,” he said.

Authorities took him to an immigrant detention center in upstate New York, where he spent the next five weeks trying to stay in the U.S. He and his fiancée even got married while he was detained, hoping immigration enforcement officials would be lenient.

They weren’t.

In late October, officers woke him up at 3 a.m. and told him to grab his things. They flew to Newark Airport and then put him on a flight to London.

Sangari stayed in the U.K. for about a week and then moved to Canada. He now lives in Niagara on the Lake with his wife, who commutes across the border to Buffalo every day for work.

That’s how the system works. It’s a dragnet. You can be a model would-be citizen, philanthropic and entrepreneurial. But if you stay in the U.S. a few days more than you’re supposed to and then you drive a few miles over the speed limit, you’re done.

Trump’s emphasis on deporting people like Sangari is new, especially given that he spent the last two weeks vacillating all over the place on deportations. But it’s been his official position ever since he released his immigration platform last August.

The candidate’s platform, which he highlighted last Wednesday night, calls for “criminal penalties” for anyone who stays longer than a visa permits. This would be a massive shift in immigration law; now, as the ACLU has detailed, simply being in the U.S. without the proper authorization doesn’t necessarily make you a criminal.

Trump would change that.

Many immigration experts find Trump’s hard-edged rhetoric on deportation of visa overstayers to be bizarre.

“It underscores his complete lack of understanding, or his staff’s lack of understanding—I’ll put it on him—of how the immigration system works,” said David Leopold, an immigration attorney who formerly headed the American Immigration Lawyers Association. “He just doesn’t understand it.”

“He’s talking about criminalizing civil immigration law,” Leopold added. “It makes no sense.”