LONDON (NYTIMES) - He could have misspelt his address, or entered an incorrect age - something fairly harmless - but no, the error Mr John Stevenson made in an online form for the US Department of Homeland Security was clicking on the wrong answer to a question.
The question asked whether he was a terrorist.
Mr Stevenson, a Scottish grandfather, said he had made an honest mistake, but it might ruin his plans to fly with his wife to New York City on Monday (Dec 3).
He ran into trouble filling out the automated online form on the Electronic System for Travel Authorisation, known as ESTA, that allows people from selected countries to apply for travel to the United States without a visa.
"One of the questions asks if you are a terrorist and it must have jumped from 'no' to 'yes' without me knowing," Mr Stevenson was quoted as saying on the news site The Independent.
When he noticed the error, he said, he called US Customs and Border Protection.
"They looked up my ESTA number and said, 'You're a terrorist.' I told them that I was 70 years old and I don't even recognise what that means," he said.
"It's terrible, it's shocking and so stupid. I don't know why that question is on the form in the first place," Mr Stevenson said.
He was not the first to have trouble with ESTA, but the system seems blind to mitigating circumstances.
Last year, a baby whose grandfather made a mistake similar to Mr Stevenson's was told to apply for a visa.
This summer, Mr Javier Solana, a former secretary-general of Nato, had his application refused because he had travelled to Iran in 2013.
A spokesman for Customs and Border Protection provided a list of the form's questions, which included: "Do you seek to engage in or have you ever engaged in terrorist activities, espionage, sabotage, or genocide?"
The Department of Homeland Security did not explain the reason for this question, and it is not clear why any terrorist, spy, saboteur or mass murderer would answer "yes."
Doing so, whether or not by mistake, does not mean that the person will be banned from the US, but it doesn't help. It usually means having to go through the lengthier and more expensive process of applying for a visa.
For the family of the baby, whose grandfather, Mr Paul Kenyon, made the mistake, the trip cost an extra £3,000 (S$5,200), including new flights, The Guardian reported.
An unsuccessful ESTA application may mean people can no longer apply for a visa waiver. But they, too, can still apply for a visa. The Department of Homeland Security did not respond last Friday to questions about the form.
Uncomfortable questions asked of travellers at the US border are at least as old as Ellis Island, the former gateway for millions arriving in New York Harbour by sea.
In the hall of the inspection centre, opened in 1892, officials asked new arrivals about disabilities, infectious diseases and whether they had been "in a prison, almshouse or institution for care of the insane".
The questions asked by ESTA and other immigration procedures have evolved alongside security concerns in the US.
The Immigration and Nationality Act, which sets out the eligibility of people to enter the country, was amended after the Sept 11, 2001, attacks to expand the grounds for denying entry to people who the government said had ties to terrorist activities or groups - and, in some cases, for denying entry to their families.
In 2016, the Obama administration tightened the rules to prevent citizens of 38 countries who had travelled to Iran, Iraq, Libya, Somalia, Sudan, Syria or Yemen after March 1, 2011, from entering the US under ESTA. It also required visa applications of people from the 38 countries who were also citizens of Iran, Iraq, Sudan or Syria.
Other requirements have been relaxed, such as the 22-year ban on HIV-positive foreigners entering the US, which was scrapped in 2010.
These rules were less restrictive than President Donald Trump's executive order sharply limiting entry from seven countries, which prevents some people from applying for visas and requires others to obtain waivers before applying for them.
Mr Stevenson may still have time to rectify the error at the US Embassy before his flight, but if not, airlines do not generally refund tickets for missing travel documents.