I don't know if this is my absolute worst experience per se, especially since I didn't actually lose any money, but last year I contacted a model called Zoi for a custom video. She's more or less retired as a nude model but still manages a modelling studio. She answered out of the blue only two months later and apologized saying she doesn't often read messages sent to her official site or her old e-mail anymore. However, she agreed to shoot the video. Like I said, her answer was unexpected at that point, and it wasn't a good time for me to spend money on a custom, so I asked if I could ask her again in a couple of months. She said sure.
So, two months later, I contacted her again. This time, she answered more quickly, we agreed on the details, and she told me she'd shoot the video a few days later. Except she didn't. I sent her a few messages asking for an update or whatever, but she never answered until a month and a half later, saying she had caught the flu. Once again, she offered to shoot it a few days later and I said ok. And once again, she didn't do it. No explanation or apology this time.
It wasn't a scam, but it was extremely unprofessional and disappointing.