It's amazing how a sample can sew up perfectly and the real thing goes to crap and nothing was changed. When I free motion quilt I triple check everything and quilt on a sample for at least 15 minutes before I start quilting on my quilt. Whenever the quilting is going great I write down the tension settings, the size and brand of thread used, the size and brand of needle, and the type and brand of batting I'm using. One small change and the nice quilting can turn ugly next time.
How did you break your machine by adjusting the tension?