Sometimes the most effective lessons sting a bit. For Nadia Caffesse, that pain came in the form of a number of tiny needles lodged in her hands, forearms and chest.

In September 2006, Mrs. Caffesse, now 45, and her family were driving through Big Bend National Park in Texas, where she found herself admiring the native blind prickly pear cactuses jutting out along the rocky roadside. One of them would make a nice addition to her garden, she thought, so she decided to ask her family to pull over so she could pick one.

She was violating a cardinal rule when visiting a national park: Take only memories and leave only footprints.

“They aren’t just pretty words,” she said. “They are a poetic threat.”

She knew she’d made a mistake the second she grabbed the paddle of the cactus. “The pain was instant, searing and, because of the diffuse nature of all those tiny needles, unrelenting,” Mrs. Caffesse recalled.

She finished her day not with a souvenir to take home, but with red, swollen arms and an enduring respect for the rules.

We often hear of tourist misbehavior, some egregious and some innocent, drawing public outrage. This year alone, a man was recorded carving his and his girlfriend’s name into a wall at the Roman Colosseum; children in England defaced a more than 200-year-old statue with bright blue crayon; and in Paris, the opening of the Eiffel Tower was delayed one morning after security officials said they had found two American tourists sleeping in the monument overnight.

In an effort to help future travelers learn from others’ mistakes, The New York Times asked readers to share examples of instances in which they’ve committed a travel foul or have acted against good tourist etiquette and, maybe, their better judgment. In the more than 200 submissions we received, one consistent theme emerged: There are lessons here.

Uninvited guests

Maybe you’ve noticed while crossing international borders just how strict the authorities can be about bringing in produce or agricultural goods.

Jennifer Fergesen, a 29-year-old food writer from New Jersey, was on a monthslong trip to various countries after finishing her master’s degree several years ago. On her way back from the Philippines, she had a layover for a couple of days in Austria. She decided to bring some fruit with her from Manila — a bag full of mangoes and mangosteens — to have for breakfast once she arrived at the hostel in Vienna.

Ms. Fergesen conducted a quick Google search and perused an official European Union travel website, concluding that bringing a few pieces of fruit for personal consumption would be fine. But she didn’t expect company at breakfast.

“As I cut open the last mangosteen, I noticed something white under the upper leaves,” Ms. Fergesen said. “When I touched it, countless baby spiders ran in every direction across the breakfast room. I crushed the mother spider but couldn’t find a single baby.”

She followed Austrian agricultural news for a year afterward, she said, “looking for word about a new invasive spider.”

A drop in the dark

Humans, unlike runaway baby spiders, can hire tour guides to help them find their way. And if you happen to be exploring dark, underground burial sites, it might be worth tracking one down.

In the early 1980s, Michael Koegel, 64, then studying abroad in England, found himself in Rome with a few friends. Near the Appian Way, an ancient Roman road, they discovered an entrance to some catacombs and decided to explore.

As the friends marched single file into the dark, illuminated by the dim glow of their cigarette lighters and a candle they’d found, they could hear, but not see, a tour somewhere in the distance.

All was going smoothly until one friend, who was directly in front of Mr. Koegel and holding the candle, suddenly vanished.

“I heard the rush of gravel and a sickly thud,” Mr. Koegel recalled. “Afraid to move, I thrust my lighter into the darkness, but saw nothing. I called out his name several times but got no response.” Finally after several tense minutes, they heard a muffled, “I’m OK.”

The friend had fallen about eight feet down, Mr. Koegel said. Thankfully, his injuries were minor.

“Being naïve is not an excuse for bad behavior,” Mr. Koegel said. “I was let loose in Europe for nearly a year at a very young age and felt invincible and above the law.”

Planter’s punch

Most readers’ confessions involved breaking rules, but a few travelers got tangled up trying to be good. It turns out that sometimes being overly polite can have consequences, too.

When Laurel Thurston, a lawyer from California, traveled to Paris one summer in the 1990s, every night the hotel host would generously offer her a complimentary, “but undrinkable” aperitif, which she stealthily disposed of in a nearby plant, so as to not offend her host.

What Ms. Thurston did not know, she said, was that this particular plant was a rare specimen, nurtured for two generations.

“Ten nights in, the plant was noticeably fading, to the host’s baffled consternation,” she recalled. “Whoops!”

Ms. Thurston kept mum about the plant’s boozy new diet, but tried to make up for it by tipping extravagantly, she said.

The kids were all right

If we aren’t going to enjoy the complimentary drinks offered by locals, the least we can do is accept their advice.

In 2007, John Rapos, 59, and his husband were in Morocco and on their way to the village of Aït-Ben-Haddou, a UNESCO World Heritage site a few hours from Marrakesh. Somehow, they veered off the not-so-clearly-marked road and found themselves driving their rental car in a dry gravel riverbed.

“Several kids started chasing our car, and we thought they were being aggressive so we rolled up our windows and tried to ignore them,” Mr. Rapos recalled. “It turns out they were just trying to direct us back to the road.”

Once Mr. Rapos and his husband understood that the children were gesturing at them to turn around, they were able to find their way back to the correct course.

“I’m not sure I have great lessons for other travelers, but I think for me, travel experiences can be enhanced by being a little more open to people than I normally am,” Mr. Rapos said.

And a more practical lesson Mr. Rapos learned from the experience: “If the road doesn’t seem right, it probably isn’t.”

Sometimes the stars align

On rare occasions, our embarrassing misadventures lead to life changes, not just life lessons.

A couple of years ago, Lindsay Gantz, a 28-year-old nurse from Buffalo, hit it off with her tour guide while zip-lining in Monteverde, Costa Rica. After spending the day together, the two went to dinner. Afterward, they rode on his motorcycle to what they thought was a secluded field to stargaze. In the passion of the moment, the splendor of the cosmos gave way to more earthly pleasures.

“We didn’t realize that the location was not so secluded until the police lights were shining down on us in a somewhat compromised position,” Ms. Gantz recalled. “Apparently there were neighbors nearby who overheard us.”

The police were understanding, she said. They took the young lovers’ information and asked them to leave the property. Now, she said, she is “extremely respectful and mindful” of laws in Costa Rica, and elsewhere.

Oh, and that charming zip-line tour guide? He’s now her husband.

Making amends

While many travel mistakes are harmless and made without bad intention, some can be more serious — even criminal.

We received some anecdotes describing instances in which someone took something from an archaeological or historical site or inherited such an artifact from a family member. (We won’t name names; you know who you are.) And it raised a question: How can I return something that was taken, and will I get into trouble?

It depends on the circumstances under which it was taken, the value of the object and why it was taken, said Patty Gerstenblith, a law professor at DePaul University and director of its Center for Art, Museum & Cultural Heritage Law.

If you are in the United States and want to return an item, a good first step is to contact the branches of U.S. law enforcement that deal specifically with art, cultural heritage and antiquities, Dr. Gerstenblith said. For example, the Federal Bureau of Investigation has a team that investigates art related crimes, and the Department of Homeland Security has a Cultural Property, Art and Antiquities Program that specializes in investigating crimes related to looted or stolen cultural property. U.S. law enforcement could help facilitate the transport and return of any objects, as well as communicate with foreign governments.

It may be tempting to mail back an improperly acquired object with no return address or drop it off outside an embassy or a consulate, but neither strategy will guarantee anonymity, Dr. Gerstenblith said. Hiring a lawyer could help ease any legal consequences.

“People may be fined,” said Dr. Gerstenblith. “I don’t know how often people go to prison for that sort of thing. And a lot has to do with whether their goal is commercial. If they pick up something with the goal of selling it, they’ll be treated more harshly than somebody who puts it in their pocket and takes it home.”

There are reasons that removing items from important sites carries consequences, Dr. Gerstenblith said.

“Everybody thinks essentially that they’re an exception, that their doing one little thing isn’t hurting the bigger picture,” she said. “But the truth is, it is. Because then everybody else thinks they can do it, too. And if 1,000 people come and they each pick up a stone off the site, or out of a national park, pretty soon there’s nothing left.”

But even as we make mistakes while traveling, the silver lining is that hopefully, we learn something valuable from the experience, or even better, it gives us a profound new perspective — which is, after all, one reason we travel at all.

“We love a keepsake from beyond the gift shop because it somehow feels more real,” said Mrs. Caffesse, the traveler whose coveted prickly pear cactus souvenir certainly felt real.

But Mrs. Caffesse realized that if she had succeeded in bringing home the cactus, it would have lost what made it so special to her in the first place.

It’s better, she said, to just leave the things that delight us exactly where they are.

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