I've got a couple!
The first one isn't terrible, just kind of disappointing:
I was working with an author on a project they were self-publishing/self-promoting. I had negotiated good compensation from her to be paid upfront before each group of illustrations would be done, she had done her research on marketing and was able to tell me her game plan, she had a contract and a lawyer for both of us to talk to--all was good on the business end.
She was doing a graphic novel style short story anthology of retellings of classic fairy tales with a sort of the-women-were-actually-in-charge-of-what-was-happening to them bent. Removing the damsel in distress trope, if you will. It was my first big freelance job and I hadn't thought to request the manuscript before signing the contract. Once I'd signed and been paid, I got the manuscript to work from and there were some issues I had with the content and how I felt women were being portrayed as sort of malicious and self serving. Lesson learned there. I was conflicted about having my name all over this project and having friends and family wanting to read it considering the content. But the author ended up having some emergency health issues and I haven't heard from them in a year, so I'm considering the project shelved.
This second one is more of a facepalm, what-was-I-thinking-taking-this-job situation:
I had another author approach me wanting illustrations for his book. There were lots of red flags in the initial meeting such as him starting off the conversation with, "I'm really looking for more of a partner in this idea than an illustrator" and he proceeded to tell me about how his idea should be made into book series' and movies and action figures.......yeah. I know. I chose to overlook these flags mostly because I really needed the money and had somehow made him understand that paying me a fair rate was non negotiable. He was clearly not professional at all and working with him was such a headache between poor communication and him not knowing anything about the industry or what he wanted from me.