Marrakesh is unfortunately the type of travel destination that - while beautiful and unique - is a place where you need to keep your guard up at all times. Some scams you don't even realize until someone informs you that it was indeed a scam. There are a variety of scams that people try to run on you and I have outlined the most common ones below.
Quick reminder: the conversion of DH to USD is about 10 to 1. So 10 DH is about $1.
The Restaurant Scam
You get to a restaurant and everything is so inexpensive! You order your food, have a great meal and then when the bill arrives the actual food prices are twice or three times what you saw on the menu. You tell the waiter and he calmly explains you must have read the prices wrong and returns with a new menu that has the same prices as the bill. Not good.
Avoid it: This scam is a bit more intense and thankfully I never ran across it, but a couple people in my hostel did. Their suggestion was to take a picture of every menu before you order so that if this happens you can show them the picture with the old prices. Even if you don't do this you can refuse to pay and then leave. They will threaten to call the cops but this is one of the most well-known scams in Marrakesh and the cops will always take the tourist side.
The Nice Guy Scam
The souk and city center are both very confusing places and this is the number one way unemployed youth in Morocco live off tourists. If you look lost and confused (especially if you are a woman), a young Moroccan will almost certainly jump at the chance to help you find your way. He might act as if he is doing it out of the kindness of his heart and may even say multiple times that he doesn't want anything in return. But be warned, they will ALWAYS ask for around 30-50 DH afterwards in exchange for their help. If you refuse to give it to them they will make a huge scene to attempt to embarrass you into paying them.
Avoid it: Either walk around confidently (even if you are lost), take the help and refuse to pay at the end or go into the "help" knowing you will be paying afterwards.
The Animal Scam
When walking around Jemaa El Fnn Square there are snake charmers and monkey handlers scattered throughout the massive space. They will both walk up to you and immediately put the animal on your back or in your hands. Once you take pictures they will then demand 100-200 DH for the encounter.
Avoid it: Always negotiate the price of any animal encounter before you begin taking pictures. 30-40 DH is more than a fair price to pay for 10 minutes of time and a couple of pictures.
The Henna Scam
It's a good rule of thumb to be wary of anyone in Jemaa El Fnna Square. A big scam in the square are older women that will literally grab your hand and start drawing a henna tattoo on you. The resulting tattoo is usually extremely poor quality that will barely last a couple of days for a very expensive price (100-200DH).
Avoid it: Don't get Henna in the square; go to Henna Café for amazing authentic henna right outside the square.
The Food Stall Scam
Again in Jemaa El Fnna Square every night there are large food stalls with tons of amazing food at low prices. But be very careful, some of the food stalls prey off of unsuspecting tourists and will bring you extra plates of food or food and drink that they say is complimentary. Beware! At the end of the meal they will stick you with a huge bill including all of the 'complimentary food'.
Avoid it: If you receive any food that you didn't order or food that they claim to be free refuse it! And don't let them leave it on the table, make them pick it up and take it away.
The Guilt Trip
When shopping in the souk - especially for rugs and antiques - the shop owners will invite you to sit down and have some tea (or berber whiskey as they call it). Once they show you 10 different rugs if you refuse to buy anything they will get angry and try to make you pay 20-30DH for the tea they offered you.
Avoid it: Refuse tea when you first arrive or just either walk away if they pester you about paying for the tea.
Bottom Line
Marrakesh (and Morocco in general) is a really interesting and unique place to explore. I hope this list doesn't deter you from going but instead makes your trip a little bit easier. While these scams are annoying and cause many people to pass bad judgement on an amazing country and people, just remember that the scammers are running these because they are trying to get by in very poor conditions. Ignore the scams and enjoy the beauty of Marrakesh.