Maybe she washed her tart burner out with water and it had some miniscule cracks that trapped water and it caused it to make a popping sound when it got hot. I don't know, but I don't think it was your tarts, it was probably her burner. I'd suggest she use a different burner. Other than that there's no reason that should happen that I can think of. Some people you just can't ever please no matter how hard you try. I had a person tell me they had no hot throw from a tart and scent that will blow me out of the house every time. If you want to be really nice send her some free tarts. Personally I wouldn't I would just explain to her that it's never happened to you or any of your testers and it's probably her warmer. If it's a bulb burner maybe she put a 100 watt bulb in it or something:) That would do it! Then again she may be just trying to get some free tarts out of you, some people are that cheap. Don't feel bad about it, I'm sure it wasn't your product. I can't imagine why that would happen...ever. Maybe someone else will have some ideas for you.