1. Stealing. Visitors steal roughly $41 million worth of towels a year in the United States, according to Robert Mandelbaum, a lodging industry consultant with PKF in Atlanta. Add filched bathrobes, ashtrays and lamps, and that figure doubles. Dishonest patrons are "a continuing problem" to the hotel industry, according to Robert Rusting, editor of Hotel/Casino/Resort Security newsletter in Westbury, N.Y. Although no one keeps an official tab on industry-wide losses, he says resorts are increasingly worried about guest theft. "Now that some of the rooms have more expensive equipment like faxes, you're talking about thousands of dollars worth of equipment rather than just a few dollars worth of towels or linens," Mandelbaum says.Lydia Ramsey, a business etiquette expert in Savannah, Ga., who has written a book called "Manners That Sell -- Adding The Polish That Builds Profits," faced an ethical dilemma as a guest at the New York Hilton. "I wanted a Diet Coke, but I didn't want to leave my hotel to buy one. Should I pay $1.50 for a Diet Coke from the minibar, or should I take it and then replace it with a cheaper one? I mean, a Diet Coke is a Diet Coke, right?" she says.She ended up taking the soda and then replacing it with an identical vending-machine can, which I think was an acceptable solution. But I've heard from frequent travelers who remove bottled water, drink it, and refill the bottle with tap water. That's unacceptable.

2. Lying. One of the most memorable interviews I've ever conducted was with a clinical psychologist named Deborah Cooper on the topic of travel industry lies. She suggested there was an epidemic of lying on both sides of the counter. What's happening? Airlines, hotels and car rental companies misrepresent the facts "because they can get away with it," she told me. And so do travelers.Dan Sondhelm, a consultant in Alexandria, Va., knows how easy it is to lie -- and get away with it. He's booked special room rates using his AAA card, but then never had to show his card. "I imagine I could even qualify for an AARP rate if I wanted to," he adds. Sondhelm, by the way, is 31. His point? "No one checks to see if you actually qualify for the rate. It's so easy to lie."It should be noted that Sondhelm has never booked a rate that he didn't qualify for, although he confesses to swiping extra shampoos and soaps off the housekeeping cart at the Fairmont hotel in San Jose, Calif. "I have nephews and nieces who love those things," he says.

3. Cheating. It's much more difficult to quantify the cost of cheating to travelers. But little shortcuts here and there can add up. In a recent survey by Westin Hotels, 40% of travelers admitted to eating more while they're on the road, while 17% said they actually gain weight. (The 23% difference? Scroll up to the paragraph on "lying" for the answer.) You have to wonder how many of those travelers are breaking their diets -- cheating on themselves, really -- when they're away. And then there are the affairs. I've lost count of how many times I've been tempted while traveling. I can't find any surveys on infidelity, and I can only guess the reason for it. And no one wants to talk about it.I attended a conference recently with my friend who I'll call Charlie, because that's his name. We had several conversations about his struggle to stay on the low-carb Atkins Diet. We both concluded that it was impossible to stick to the good doctor's diet when we were away, not with our hosts trying each night to outdo themselves by serving us increasingly exotic, and carb-rich, dishes. Charlie gained six pounds during the conference. I packed on one or two myself.