Race spectators can be a mixed blessing. From your parents waiting for you at the finish line of your local 5K to the million plus fans who line the New York City Marathon route, they can give you a boost when your legs and brain feel too tired to go on.

“They provide inspiration and support and excitement and enthusiasm for runners,” said Phil Stewart, editor and publisher of Road Race Management and race director of the Cherry Blossom 10 Mile Run in Washington.

But spectators can get in the way too, whether on purpose or not. At last year’s New York City Marathon, I had to navigate my way around a woman who jumped into the course between miles seven and eight and ran with us in jeans and high heels so she could take a picture. Near the end of the race, a spectator reached out from the sidewalk to give me a high five and instead caught my hand and wrenched me back by the shoulder.

It can happen to anyone, anywhere. Perhaps most famously, in the 2004 men’s Olympic marathon in Athens, the Brazilian runner Vanderlei de Lima lost his sizable lead when he was tackled by Neil Horan, a defrocked Irish priest who has made something of a minor career out of interrupting sports events. Mr. de Lima finished third.

While running the 2014 Ghost of Seattle Marathon, Robert Watson was pounced on by a dog who left a few scratches on his stomach as a race souvenir. “The owner didn’t bother to even apologize,” said Mr. Watson, who still managed to set a personal record for the event.

In the 1998 Chicago Marathon, Matthew Riegler yelled at a clump of spectators who appeared to be blowing cigarette smoke in the direction of the runners for fun. “I swore at them and resumed my misery,” he said.

And last year, as Hillary DeLong neared the finish of the Chi Town Half Marathon in Chicago, she

had to navigate her way through the crowds that had stepped off the ice-covered sidelines right onto the course, which was less icy. “At this point, I had spent 13 miles hyper-focused on not slipping and tearing or breaking something, but then had to weave pretty aggressively around entire families just to get to the end,” she said. “I usually try to give spectators the benefit of the doubt and a lot of liberties because I know how hard it can be to be out there cheering someone on, but this was next-level unawareness.”

Road Race Management doesn’t offer race directors specific guidelines on handling spectators, but managing them is always tricky, said Mr. Stewart. “It’s one of those things where, yes, they need to be controlled. But balancing that versus the excitement and exhilaration they bring — how do you strike a balance?”

For exceptionally crowded races, or in extremely congested race areas, the best line of defense is barricades, said Susan Harmeling, executive director of the Gasparilla Distance Classic Association, which puts on four races in two days in Tampa, Fla. They put up barricades at the start and finish of the course to keep runners “at a minimum of six feet from any spectators,” she said.

Near mile 11 of the Gasparilla Half Marathon, where children from the Florida Blue Kids Running Program cheer runners on, race officials instruct them to stay on the curb and put their hands out but close to their bodies so runners can come to them — and not the other way around, said Ms. Harmeling.

But things still happen. For most races, it’s impossible to have barricades or race monitors along the entire route.

In places without barricades, Ms. Harmeling said they rely on spectators to use common sense and for volunteer coordinators to intervene if anyone interferes with the race, which often resolves the problem. When runners let an on-course volunteer coordinator know that a spectator was blowing cigar smoke into the racecourse at one of their 2013 races, for example, the coordinator got him to put out his cigar.

Desiree Linden, who ran on the 2012 and 2016 United States women’s Olympic marathon teams, is a fan of spectators. “They’re just your go,” she said. But when one tried to coach her from the sidelines during the 2012 Olympic trials, telling her “You have to be running faster if you’re going to win,” she decided to laugh. “I was thinking to myself, ‘I’m running really fast, and this is really hard.’”

Indeed, humor can sometimes be the best antidote. When the elite runner Matt Llano was racing in a tight group of eight at the front of the USA Track & Field 20K championship in New Haven in 2014 — the group was running at a 4 minute, 40 second-per-mile pace (translation: very, very fast) — they were joined by an unexpected guest: a woman in flip-flops and plush pants carrying an overfilled plastic shopping bag who ran with them for about five seconds before dropping back. “It was a nice moment of comic relief in the race,” said Mr. Llano. “We joke about it now and call her the red velvet pants lady.”

In 2015, Francis Pardo traveled from Bogotá, Colombia, to Colorado to run the Leadville Trail 100, a taxing 100-mile ultramarathon. Near mile 80, Mr. Pardo was greeted by a handful of spectators wearing nothing but glow sticks and the skin God gave them. They offered him potato chips, soda and whiskey; he took the first two but passed on the booze.

“They were really cheerful and enthusiastic,” he said. “For me, it is one of the highlights of that race and part of the whole Colorado 100-miler experience.”