"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."

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.
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.
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.
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."

Kevin Winter / Getty Images
Internet service providers
"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."
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."
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."

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."