I'll go first. Early in my career, a gallery in Vienna offered me a show. Nice space. Nice people. They just needed me to pay for shipping both ways, the opening catering, and their "promotional materials", which turned out to be a photocopied A5 flyer. I said yes because I was young and wanted the line on my CV. The show happened. Nothing sold. They asked me to cover the "storage fee" for the work that hadn't moved. I paid that too. What's the red flag you saw early, told yourself it was fine, and then it absolutely was not fine?
