Yes, a whopper. It was at a casino. A 2-person shop, stopping at a restaurant for a meal, drinks at the slots, and one of the card tables. The payout was $90 (2016) plus up to $100 in gambling money, and all meals and drinks reimbursed.
We got there, did all the things one by one. Spent about $90 gambling, another $75 on the meal, plus another $30-40 on drinks. Each time I had an interaction, I surreptitiously recorded it on my phone for later review. We were there for at least three hours. After one of the last interactions, someone behind us informed them that I was using my phone to record, and then she walked away. The person I was talking to told me that it was ILLEGAL to record anything inside of a casino (not only did I not know this, but the shop instructions specifically told me to record everything), and they made me delete all of the recordings one by one. Then they said they were getting the police to arrest me. Serious suspicion and worry on their part. I immediately said, "No, wait, I'm a mystery shopper! I have to record these interactions!" They were still suspicious, but thankfully let me go.
I did the evaluation anyway (doing my best to remember the details of the interactions that had been deleted), and submitted my receipts, noting that I'd had to reveal myself to one person due to not wanting to go to jail, but that no one else knew it. They immediately removed me from the project and banned me from ever doing casino shops again. Not only did I not get the $90, but I also lost the hundreds we had spent on food, drinks, and gambling.