When you spend a few years working in a certain industry, you discover some dirty secrets that would have horrified you when you were applying (and still horrify the friends you tell to this day).
A recent Reddit thread asked people to name the "dirty little (or big) secret" about their industry they thought people ought to know. 
Some of these stories might even help you save some money from businesses that try to trick you.
Since we can't verify the identify of the Redditors or their employees, some stories should be taken with a grain of salt. 
Here are some of the best dirty industry secrets that came out: 

Bookstores

Big chain bookstores throw massive amounts of books away. Via Reddit user allosaur:
"Mass market paperbacks are cheap to manufacture and get shipped out in huge volumes. For some publishers (particularly ones that put out new mysteries or romances quarterly) when the bookstore wants it off the shelf to make room for something new, it's just not worth the cost of taking them back and finding someone else to sell it ... So as a bookstore employee I spent hours ripping the front and back covers off of books, then tearing the book at least in half so that no one could read it later. The covers get sent back to the publishers, and the books that could have been donated to a library or school get put in a locked recycling container out back. A manager had to come back and check my work to make sure the books were not left intact.
I almost cried the first time I had to rip up a load of kid's books (in a city with high child poverty rates and underfunded schools)." 
Discarded books
flickr/auntlaura

Funeral Parlors

If you have a strong stomach, it's worth reading this full (and graphic) comment fromRedditor arrghbrians who argues that the mortuary business convinces many people to go through incredibly expensive procedures that aren't really needed.
"..in most states, the law only requires embalming if you are transporting a body across state lines or are not planning to inter for more than 72 hours and/or having a public viewing. It has not a single thing to do with public health. It’s a cash cow, plain and simple. It is barbaric, costly, and does not keep the body from deteriorating. But we’ll tell you just about anything you need to hear to get you to agree to it."
Funeral home casket
Chris Hondros/Getty Images

Horticulture/farming

"In the horticulture industry don't eat anything without washing it and (when) buying fresh plants ie tomato plants always wait at least 14 days before eating from the plant. Most with holding periods are around 14 days, the shit we spray for everything is quite nasty. We have regular blood tests to make sure we don't die pretty much." 
From Reddit user geekmuseNU:
"I work on a farm. When they say you should wash your produce thoroughly at home, they're not joking." 

north carolina farm strawberries
AP Photo/Gerry Broome

Oil Change

At a national chain like Jiffy Lube, there's going to be a huge variety in management and the level of service. 
However, according to Reddit user jmhoneycutt8, who claims to be a former manager, half of the services offered or charged for don't even get completed. There's incentive to tack on services that there isn't time to complete because employees' hours are determiend by "average ticket sales."
The thread has many more horror stories.  
Jiffy Lube
flickr/storem

Hotels

Rates are not as set in stone as people think. From Reddit user schlotzy4:
"After working in the travel industry I can tell you that hotel room rates are often not fixed prices. If guests come to the front desk and ask the price we generally start at the high end. Most people accept this as fact and pay up. However, if a customer is hesitant or threatens to walk out we can sometimes drop the price to keep them there. Often there is a bottom line price set by the owners- we can't go any lower than that or we lose money."
And be wary of third party sites. Via Reddit user paintedroses:
"NEVER BOOK THROUGH A THIRD PARTY (Expedia, Hotels.com, Priceline, Orbitz, etc.). Hotels overbook all the time, and your reservation is the first to go in a "sold out" situation. Third parties guarantee you a room, but not necessarily the room you booked or even a room at the hotel. We can "walk" you to another hotel that has vacancy. Third party requests are also not something we have to honor or even try to honor. "
If you find a better rate on a third party site, your best bet is to call the hotel and ask if they can match it. 
Motel
flickr/caveman_92223

Carpeting

Via Reddit user SlightlyStable:
Little secret. When you buy carpet you usually need less square footage of pad than carpet. But most companies bill you the same square footage for both and often send less pad than you paid for.
Some more color from the thread — carpet is usually ordered in excess of the square footage required for rooms that aren't the exact length/width of the rolls you order to minimize seams where pieces meet. People don't' see padding. It doesn't matter. 
Carpeting
flickr/juhansonin

Smaller award shows

Via Reddit user bridow
"I'm a celebrity event photographer in Hollywood. Most of the smaller award shows winners like the MTV VMAs, Teen Choice Awards, etc...already know they are going to win. This motivates the talent to come to the event. During the show they are backstage talking with friends and take a seat during a commercial break just before their award is announced. The few exceptions are the Oscars and Golden Globes where the audience is mostly celebrities."
kanye west 50 cent vma mtv
Kevin Winter / Getty Images

Internet service providers

Via Reddit user static74:
"Fiber Internet Service Provider here - bandwidth is not a scarce commodity like they want you to think it is. It is all about profit margins and over subscribing the network."
User bigdonkey adds " Most of the bottlenecks are the result of the ISPs not building out their local networks to meet demand." 
Comcast Truck
flickr/scriptingnews

Catering

Via Reddit user bananabilector:
"When you order catering, we know we can't trust you to have your guest count right, so the kitchen massively over produces for your event. The staff then eats their shift meal from your order and the rest gets tossed. Sometimes it's only a little, sometimes it's 200 plated mains or 15 hotel pans full of potatoes and green beans. And at $13-35/plate depending on the menu, we still come out on top."

Catering
flickr/Elvert Barnes

Delivery

Via Reddit user JamesW89:
"I work for a UPS store. Here is a few things I have learned since working here...
Writing fragile on your package means nothing.
Your package WILL get thrown around, dropped, and beaten up; if it is breakable then according to our guidelines for properly packaged items it needs to withstand 1000lbs of pressure and a 4ft drop.
UPS capital claims is terrible as well they will do whatever they can to not pay you the amount you insure your package for."
UPS truck
Getty Images

Fine dining

Via Reddit user SG804:
"Fine dining cook here. 30% of your meal is butter. That's why it's so good." 
Chef butter
flickr/rdrpr