I once sent my machine to the shop to be repaired...the nice man called and explained that I hadn't threaded the bobbin into the case properly. Just that one thing and the machine was breaking thread and sewing tangled knots on the back of fabric!!! Didn't have the instruction book to check since it was a really old machine. I'm sure though that we weren't the first or last ones to do this. We live and learn and will make some other mistake next time. LOL