LONDON — He could have misspelled his address, or entered an incorrect age — something fairly harmless — but no, the error John Stevenson made in an online form for the United States 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.

He ran into trouble filling out the automated online form on the Electronic System for Travel Authorization, known as ESTA, that allows people from selected countries to apply for travel to the United States without a visa.

“One of the questions ask 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.