106 Dirty Business Tactics That You May Not Know Exist

Fake reviews.
I worked for a startup that had a sleezy CEO and got most, if not all, of their business by fake Yelp, Google, Glassdoor and other review sites in our industry. CEO was a compulsive liar and had no morals.
It is easy to see fake reviews now since they are usually a bit more eccentric and polished and I have lost all faith in them.

Try to make it seem like you're going to get some form of extra special deal out of it.
E.g "2 for £10!!" offers on products that are £5 each anyway.

Bottled water. Much of the water is from public sources and is marked up hundreds of times over. I just bought some after a flight and paid more for a liter than I would for a gallon of gas.

If you ever get a demonstration of a service from a company they will always use their very best, most experienced staff but once you've signed up you might find you've got the dregs that they couldn't foist on anyone else working for you.

Signing people up for shit as addons to an existing bill and hoping they don't notice the extra charges.

Maybe not dirty, but incompetent contractors will often way underbid jobs. So if you request a bunch of quotes, and all the bids are relatively close except for one that's way lower, there's roughly a 100% chance that guy will screw it up and you'll have a nightmare on your hands. Sometimes you get what you pay for.

Online shopping: Don't trust product reviews and things like amazon best sellers.
Companies have started to put a lot of marketing effort to get their products good reviews and on top of best seller lists.

A local lawn maintenance business takes advantage of unsuspecting customers in 3 ways:
On monthly bills, they double the state tax (instead of being, say, 6%, it'll actually be 12%, if you check the math).
Without discussing it with homeowners, they charge double for "double-cuts" when the grass is a little taller in areas than usual. So, if you had agreed to pay $50 per mowing, the monthly bill says $100 for each visit. They never ask - they just do it and charge double (in most cases, it's just a small "patch" of the yard that has taller grass, not the entire thing).
They're supposed to mow once per week. But without telling customers first, they start mowing every 5 days - which means they get to charge for more mowing visits per month than necessary.

Nominal weights and measures that don't match actual weights and measures. My company sells by the each but each item has a nominal weight. We intentionally produce our product approximately 10% light to save raw material costs.

Try, try, try to stay sober enough to successfully challenge any erroneous charges on your drink tab. Sometimes if a bartender or server is dealing with a drunk/heavily buzzed customer, they'll charge them for more drinks than they actually consumed. Unfortunately, few people excel at doing math when they're drunk/heavily buzzed.

I work with a lot of different body shops, and in my area the big thing to do is to enhance the damage to customers cars. I don't mean they try and negotiate harder, I mean they actively create more damage to previously undamaged panels, usually in ways that don't affect the function and are hidden to the customer, in order to get the insurance company to pay them more money. In my area, while not every shop does this, I would say the honest ones are the minority.
And it screws the customer over in the long run. The shop might promise that they will "save you your deductible", but in the end you end up with a car that has unrepaired damage or you have to pay out of pocket if the insurance company catches the shop enhancing. And the sad thing is there is almost no way to know which shops do this when you pick a shop.

Sellers targetting retired people. To them, retirement means "old" so potentially easy to trick. They send offer for "retired only" by mail or phone. You're supposed to have win a coffee machin or toaster. You come to the shop to take your prize and thay make you try couch, wine or else, using all their technics to make you think it is a good deal, proposing staggering plan with high interest rate to people would answer them they can affort to buy their stuff. And that shop disappears after few weeks before too many kids come to complain that their parents got swindle.

At theatres the price of medium cup of popcorn is usually very close to the price of the large popcorn making people more likely to buy the larger one.

When you go to buy a used car some dealers will have unrealistically low prices for some of their cars that's lower than their actual value. This is to attract you in, and then once they have you in and you're considering buying the car, they'll add what is called a "dealer prep fee" that will range around 500 dollars. This is just a fake fee they will use in order to make up for lost profit for putting the attention grabbing low sticker price.

Not sure if this counts but at my local store they have packets of candy that's often on sale as 2 for $4, but they always make sure to cover up the original price... which is $2 each

When moving out of a rental apartment/house make sure to take lots of pictures and ask the owner/landlord to do a walkthrough with you. Video the walk through. That way if they do not give you all your deposit back you have something to take to court VS a he said he said which you generally lose.
When you turn in cable/internet equipment make sure to get a FULL receipt showing what they took in and the date it was received. Scan this and e-mail to a couple different e-mail accounts. Comcast and others are bad about "losing" the equipment a couple years later, after you forget, and then billing you. CYA!!!

Jacking google business pages. Basically if a google business listing isn't claimed and controlled by a company, a competing company can weasel their way in and direct people to their business by, say, changing the phone number.
There was an article recently about how drug counselors in the Philadelphia area had it happen to them. Their listing phone number was changed to an 800 number, which directed callers to an inpatient rehab facility in Florida. It was discovered when one of these counselors started noticing his patients suddenly not showing up. He called one of them and found out he was at this facility in florida.

Offering a great deal over the phone to get you to buy or upgrade, then refusing to acknowledge the deal later on because there's nothing in writing.

Customer wants Product X from a European Manufacturer.
But Product X from Europe is too expensive. Profit margins will be low.
So we just buy Product X from some no name Chinese Manufacturer and switch labels.
These are high volume products that contractors send out tenders for. It's an open secret anyways. The contractor person in charge knows all the shit. Everyone vying for the order are ready to hand the person a cut (a bribe basically) for the order. So ultimately the client is the one getting screwed, but even he knows what shit goes around, because he ends up saving big time by paying Chinese prices for "European" products, which he goes on to sell to individual customers.
And honestly, every European manufacturer has a factory in China. Even if you get a legit European product, it's from China.

Whatever your employer says is not true if you can't prove it.
My employer started putting random clauses into my contract verbally. They didn't exist in writing.