Working undercover can yield very disturbing results.
Secret Shoplifter

Working as a secret shoplifter for large retail chain. I would basically test loss prevention and see what I could get out of a store. The total would be tallied up and taken off the store managers bonus. So they had to be on their game. One thing I used to do was steal copper electrical cable sold by the foot. I would drag it to the fence and spool it up in my car parked on the other side. This was fun to me and an obvious catch for the store. I go outside to the wire rack and there is the store manager getting a blow job from a cashier. They spotted me and quickly went back inside to leave me alone. I then went ahead and spooled out the wire as planned. Not one but the entire inventory to the point my car started to get lower under the weight. I went in and bought something and went to the cashier I saw with the SM. I asked her how long she had been there. She replied, “for a couple of weeks now, its just part time until I go back to high school for my junior year.” I then went back to the SM and identified myself and what I saw in private. The cashier was the daughter of a friend of his and is only 16. He was 37. I gave him two weeks to find another job or I would be back and blow it all up. He not only left in one week but left the state entirely. Hind sight I should have called the police but I didn’t. This was about twenty years ago. (Source)
Asset Protection Specialist

Pretty late to the party, but I was an undercover “Assets Protection Specialist” at Target for a year in my early twenties. The main taboo thing I noticed – and one of the reasons I wound up hating it and ultimately quitting- is the whole security industry is racist as f*ck. Everyone from the uniformed guards at the door to the police that did the arrests. We had policies in place to prevent targeting minorities, but it didn’t do anything. Nearly every time we called the police to arrest a black person, they would at minimum get tackled. Even if they weren’t resisting. We had one white guy who was addicted to heroin that we had caught stealing video games several times, and had even punched my boss, and he was arrested so gently you would’ve thought he was made of porcelain. That job wore on my soul, for real. (Source)
So Corrupt

Did physical security for a while, was on a temp contract with a school district for about 6 months. Witnessed a cop getting BJ’s from a 14 year old girl that I knew was a freshman and was just trying to make friends. I’d chat with her and the other students as I’d walk around on my patrols. So I stealthily recorded the act (it recorded to an SD card) and reported it to my superiors but here’s where it gets crazy. The day after my report, I was arrested and charged with ‘obstruction of justice’ and ‘production of child pornography’ and was thrown in a cell for a few hours. They kicked the sh_t out of me quite a bit (lost a few teeth) and refused me access to a phone to contact a lawyer.
I was able to finally get a kind (female) police lady to let me use the phone and get the lawyer to bail me out. Case is still ‘pending’ but holy sh_t if that wasn’t insane. Luckily for me, the phone & SD card were encrypted and the police couldn’t do sh_t about it. I’d also been fortunately paranoid enough to send a duplicate on a USB drive (unencrypted) to a friend ahead of time and asking them to hold onto it just in case. They had no idea what they had O_O. There’s a reason that whistleblowers and law enforcement don’t get along. Next time I’m just throwing that sh_t up on WikiLeaks and letting the media frenzy handle it instead. Not ever EVER worth the sh*t I went through.
I completely quit physical security after that and did only IT security from that time onward. It is quite less hassle. Source)
Social Engineering

So I can answer this one. I do Social Engineering for financial institutions (Banks, Credit Unions etc). I went to a credit union in Texas where the entire place failed on a miserable basis for security. I walked in with a fake badge that stated I worked for a made up company and I was there to an inspection of the building. I dressed in a polo and khaki pants with matching colors to my badge and walked in to the front desk. The girl there was probably in college or just out of school. She immediately let me into the back room and I walked into offices and desks that were unoccupied but located in rooms with other employee’s. I walked up to empty computers in use and plugged USB drives in, huge no no, and began typing random things into computers and taking pictures of myself at the computers. Employee’s would literally look at my and go back to their jobs without thinking anything of a guy taking selfies at their friends work desk. Once I had been in every office I went to the Vice Presidents office and opened her desk and looked through files to find personal peoples information, found tons. I went into the file room and took personal loan documents off the shelf and took pictures of myself accessing them. When I was done I walked to the person who had contracted my company and laid out all the information I had found and all the things I had done and the guy just sighed. This scenario has happened a couple of different times, most places fail somehow. Sometimes its fun and interesting, sometimes its boring and there’s nothing worth staying. (Source)
The “Ghost Passenger”

Worked as a “ghost passenger” for an airline. Basically I flew around and graded flight attendant’s performance. While still parked at the gate with the door open one flight attendant began berating passengers who were using their smart phones (which was completely permitted at the gate). She threatened to have the person in front of me thrown off the plane if she didn’t put hers away. She also threatened me. After we left the gate and were taxiing to the runway I looked back to see her sitting in her jump seat….using her phone. Took a photo of it and included that in my report. She initially denied using the phone and claimed I was being unruly on the aircraft. After she saw the photo she changed her tune. (Source)
How Sad

I work as a researcher studying illegal wildlife trade. In markets, I have seen critically endangered animals being sold openly for pretty low prices. I’ve also seen plenty of animals dead or dying in these markets with no food or water in the baking sun. But worse than that, I have friends who have had to watch slow lorises having their teeth pulled out with nail clippers and not been able to do anything. (Source)
Undercover Prisoner

Helped manufacture methicathinone so that I could get arrested with the suspect’s and spend time in the cells to discover how the drugs were being brought into the prisons. Was recruited due to my work circumstances, age and environment; had to change “states” when I was finished with the deal. (Source)
The Coercive Manager

I am a private investigator, contracted by Government to check massage shops to see if they’re soliciting. I usually get sent to shops which have had multiple complaints and an investigation is in its final stages and they need evidence to support their case.
So, I go to one place, everything seems normal except for being asked to strip bare for an oil massage. Eventually I am asked to flip over, therapist without asking just starts rubbing some “gel” on my ‘parts’. At this point, I am obligated to refuse and finish the session. I ask her to stop but halfway through the sentence I am hit with the most intense pain. Turns out, she was new and was unaware that alcohol gel is not the same as massage oil. Long story short, manager was convicted of coercing staff on the basis of their visas being revoked if they didn’t comply. And I had to stand in court and tell the events of that day. (Source)
Former LP

Former Loss Prevention here (best job ever by the way). Witnessed so much sex in the stockrooms, couples, singles, trios, you name it. We had cameras everywhere of course, especially in the stockrooms due to rampant employee theft. We had countless hours of employee sex on tape. We couldn’t reveal the cameras, which would tip off the thieves, so all we could do was watch. (Source)
John After John

I’m a TV Producer. I was working on a popular documentary a while ago. We were following a woman who was addicted to heroin and had resorted to prostitution in order to make money to support her habits. One day, a friend of hers got thrown into jail. She decided she needed to work really hard to bail them out. So she called her friend who owned a crack-house that he let girls like her turn tricks in at about $10 a pop. To answer OP’s question, I witnessed this girl share needles with many people and watched as john after john came into the house and disappeared into the bedroom with her for a little bit. (Source)
Secret Shoppers

Was secret shopping at a high end restaurant, around $100 per plate. The busser came over and re-filled my water glass (that I had already drank out of) and over filled it, so he poured part back in the pitcher from my glass and proceeded to re-fill everyone else at the table from the same pitcher. I really wanted to throw a fit, but could only include it in my report. Ewww (Source)
Never Went Back

Former Private Investigator here.. The worst thing I ever witnessed while undercover at a factory was a man who would make the rounds selling pirated porn DVDs. No big deal, until I found out he was also selling child pornography that he himself had created.
I didn’t have to “blow my cover”, rather every day I would report to my superiors who would in turn report any illegal activity to the police. After about a week of gathering evidence, the police came in and arrested 4 people – myself included. We all went to jail together and got separated from there. They only arrested me to not ruin any cover I had. I quit after that. The whole thing was too much. Never went back to being a PI. (Source)
Terrible Game

I worked as a secret shopper for a while for a small store. The most egregious thing i found were employees would hide inventory if they knew a sale was coming up. I’d also go in and ask questions about what equipment I should use just to test their general knowledge. Most failed but they were young kids getting paid sh_t so I felt bad squealing on them. I also forgot to add that I had to put up with being hit on by teenagers and creepy older guys. I had to play along and see if they knew their sh_t despite throwing terrible game. (Source)
What Happens At E3…

when I worked QA for a video game publisher, they had me work an E3 as a secret player, basically I got to play new games for that publisher and act like they were amazing in front of press. while I was taking a break I saw one of the producers hooking up with a booth babe. he didn’t recognize me, but I new he was married and had a kid on the way, I didn’t say sh*t, he got laid off when the rest of us did about a year later. but that’s just kinda how E3 goes. (Source)