The Iowa State Fair is one of the most famous stops in a presidential campaign, known for delivering the kind of memorable moments that can define a candidacy.

The fair, which attracts about a million people over ten days, amounts to a political obstacle course for candidates, who must woo voters in unscripted interactions, flip a pork chop for the cameras, deliver their stump speeches in a public forum and — most treacherously of all — eat fair food while avoiding unflattering photographs. It all happens before the hundreds of thousands of Iowa voters visiting the fair, throngs of reporters and banks of televisions cameras.

And it can easily go very wrong.

In 1987, Joe Biden lifted passages of a speech by British Labor Party leader Neil Kinnock without attribution, adding to questions of plagiarism that ultimately lead to his early withdrawal from the race.

In 2004, John Kerry, one of the country’s richest lawmakers who had been struggling to show he connected with regular voters, ordered a strawberry smoothie — a choice that had his aides scrambling to find a corn dog.

And in 2012, Mitt Romney responded to a heckler with the line, “Corporations are people, my friend.” The comment came to be a shorthand for Democratic attacks that he sided with business over American workers.

These kinds of moments can create narratives that become cemented in the public perceptions of the candidates — even when the facts may be slightly off. In 2007, Senator Fred Thompson, who ran for the Republican nomination, traveled the fair in a golf cart and, allegedly, $500 Gucci loafers. Years later, Mr. Thompson insisted that he did not own the shoes.

This year, all the major Republican presidential candidates, except former New Jersey Gov. Chris Christie, are expected to attend. The biggest showdown is expected to be on Saturday, when both former President Donald Trump and his major rival, Gov. Ron DeSantis of Florida, will be circulating through the fairgrounds.

Mr. DeSantis will participate in a conversation with Gov. Kim Reynolds of Iowa. Mr. Trump is skipping the organized events to attend with an entourage of endorsers not from Iowa but Florida — a dig at Mr. DeSantis. It’s the kind of unconventional approach Mr. Trump has taken to the event in the past. In 2015, he caused a media frenzy when he landed his helicopter near the fairgrounds and offered rides to children.