Well said Tersta!
What you described happened to me a lot and because I didn't get any feedback I never even knew there was a problem with my old models.
Lots of buyers bought them and returned them but if I had known about it I could have corrected the issue or at the very least offered them something else for their trouble.
Now I have become paranoid about my models and have been trying my best to test all the new ones I upload many times until everything works flawlessly.