I have a few, but the worst was stabilizing blanks for someone. He sent me some of his blanks and wanted them stabilized and dyed. He'd seen (and even bought and turned) some of my really awesome blanks. He was expecting the same results from the material he sent me. Half of it wasn't really suitable for dye work. The other half was lightly figured, so it definitely wasn't going to have that pop that he was expecting. And he wanted no two blanks with the same color combination. He was very unhappy with the blanks but wouldn't understand that it all boiled down to poor choice of material on his part. I stopped doing custom dyed stabilizing work for others - and decided from then on that any dye work would be with my own material that I sourced (so I could have control over that) and in whatever colors I decided to do. I will still do clear stabilizing work for others on occasion, but dyed stabilizing work I will only do on my own terms. (And I haven't done any stabilizing in a while, in part because of this - but the chambers are getting loaded up this week as I've got a few gallons of resin to use up and some material I want to stabilize.)
The deadline thing also gets me. Good work takes time. And good work isn't cheap - and I've usually sold my stuff too cheap. But this is just a hobby for me and I often get very busy with family and work that sometimes projects have to get set on the back burner for a while. So I'm doing less and less work for others as time progresses. The last person who approached me about custom work, I actually directed them to someone else who was, honestly, more set up to do the specific task than I was.