I'll start off with my recent one..
Someone liked something I had done, so they initiated a barter offer in exchange for a limited quantity. So I spent the next couple of days, making them, using my supplies and mailing them off. They are EXACTLY as pictured, in fact they look like originals... the only thing, they need to be attached differently than what the individual assumed. The method needed would only cost the person $1.50 of which they would only use less than .25 cents of the product and it would functionally behave NO DIFFERENTLY than what he originally assumed. So now I'm out my time, effort and supplies.
The person offered to send them back, but what would I use them for? I think I'm done doing things for people. Stuff like this has happened before, and I'm done wasting my time and eating the associated costs, no matter how much it is. It sure is discouraging. It makes me wonder how many others no longer help others and how it affects the hobby.