There are things customer service representatives will never tell you. That’s where former employees come in.

Former and current workers revealed some of the biggest secrets of their industries on a recent Reddit thread that has, over the last four days, garnered nearly 25,000 comments. When it comes to making customers happy, some industries — cable TV, internet service providers or health insurers, anyone? — can never seem to catch a break. (Spirit Airlines recently promised it won’t be the least popular airline in America, at least among those who use it.)

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But the participants in the Reddit thread dig a lot deeper than that. With some minor edits for style and clarity, here are the top five scariest industry secrets from that viral thread:

Don’t get ice in your drink on an airplane

From a flight attendant: “Don’t get ice in your drink, don’t drink coffee, tea, or hot water on the plane and don’t touch anything in the lavatory with your bare skin. The ice is put in a tray with a scoop and the trays don’t get cleaned very often. Every surface on the plane is touched by hundreds of people daily and not often disinfected. We don’t have the opportunity to wash our hands at all during the beverage service.” Standards for disinfecting planes vary according to time and class.

The water may be clean, but the flight attendant’s hands and equipment may not. “Between ice scoops, I’ll probably touch a seat, a Coke can, my tablet to charge someone for a drink, their credit card, a tray table and the car,” the flight attendant writes. “So basically my hand snatches up all those lovely germs and then goes back into the ice drawer to pick the scoop back up and do it all again.” Several studies have confirmed bacteria in water served on airplanes.

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While airlines follow their own individual protocols for aircraft cleaning, the safety and security of passengers is the highest priority, a spokeswoman for Airlines For America, an industry trade group, said. “As such, airlines work continuously to keep planes clean and Airlines For America and our member airlines remain in close contact with the Centers for Disease Control on actions the U.S. government is taking regarding any potential health concerns.”

Clean drinking water is also a priority, she says. Airlines typically provide bottled water while also ensuring that water available through the aircraft onboard water systems is safe, the spokeswoman added. “Airlines work closely with the Environmental Protection Agency to ensure that water received from municipalities for onboard systems is safe and to maintain that safety by following rigorous sampling and management requirements once received,” she said.

Never click the “unsubscribe” link on emails

“When you click ‘unsubscribe’ in an email they will unsubscribe you from that list, but they will add you to another because the Can-Spam Act of 2003 says you have to remove someone from your mailing list if they request it and that you have to have an unsubscribe link in your marketing emails,” one poster wrote. Marketers must honor that request within 10 days.) “But it doesn’t say you can’t immediately subscribe them to another mailing list.”

But for unscrupulous scammers, this also acts as a confirmation that this email address is active and there’s a human being at the other end. In marketing terms, that email is now more valuable and could, in fact, be added to another list. “It’s usually an indicator to increase the level of things that they’re actually sending to you,” Marc Maiffret, a Phoenix-based former hacker and digital security expert, told NBC Chicago. Experts recommend marking such emails as spam instead.

Hotel comforters don’t always get changed

“I worked in a hotel and, while we did wash the sheets and pillowcases between guests, the comforters and blankets were a different story,” one former hotel worker wrote. “They were washed just twice per year — once in the spring and once in the fall.” Another former hotel employee agreed. “We were all discouraged from changing the top comforter unless it was visibly soiled as we only had a handful of spares and they were in very poor condition.”

“ ‘While we did wash the sheets and pillowcases between guests, the comforters and blankets were a different story. They were washed twice per year: once in the spring and once in the fall.’ ” — One former hotel worker.

“In some hotels the housekeepers do something called ‘popping the sheets.’ They wipe off crumbs and straighten the sheets out to make it look like they made it with fresh linen. However, this is a big thing that most good managers look out for and the employee can get fired if they are caught.” (The industry trade group, American Hotel & Lodging Association did not respond to request for comment on policies for changing sheets or comforters.)

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While it all depends on the quality of the hotel, it’s not just the bed sheets. The online calculator Travelmath.com sent a team of nine researchers to 36 different hotels to examine hotel rooms for bacteria and concluded that, the on average, hotel rooms appear to be dirtier than a typical home, an airplane, and even a school. They looked specifically at bathroom counters, remote controls, desks and phones. The good news: These surfaces, unlike sheets, can be disinfected easily.

Mechanics use Google to figure out car problems

This mechanic wrote: “90% of the research that I do to figure out what is wrong with a car is Google and YouTube.” His advice: “Become a member of a couple of forums for each of the cars that you own. You will save a lot of money and those people are there to help. And there is a forum for everything. Just Google what type of car you have and add forum to the end of the search.” After diagnosing the problem, however, they may be the only ones who can fix it.

In 2014, ABC’s “20/20” news show reported that some mechanics to squeeze more money out of customers by doing unnecessary repairs, given the small profit margins at repair shops. He said honest mechanics are sometimes pressured by the managers or owners of shops to find problems. (The Automotive Service Association, a trade group for mechanics, has a code of ethics and says the majority of the service repair industry is ethical and don’t charge drivers for unnecessary work.)

Child-care providers don’t report “firsts”

Finally, some good news. Sometimes, the secrets workers keep from customers have the best interests of the client at heart. “Your child may take his first step or say his first real word while at daycare. But we aren’t going to tell you that,” according to one Reddit commenter. That, most people agree, is a good thing. “This is just kindness, plain and simple. You’re eliminating the potential heartache of parents who have to work versus letting them believe they saw the milestone.”

One father responded that he witnessed his child take her steps after the daycare worker saw it first: “I had a day care worker tell me she got to see my daughter’s first steps, I told her let’s pretend that didn’t happen. Then my daughter walked in front of me two days later and my wife was out of town that night, it ended up being a week later when my wife shouted from the next room that she took her first steps. She will never know. I can never brag about this to her.”