(and a gasp/giggle for non-quilters when you see the photo at the end!)
On the weekend, DD and I completely cleaned the Brother PQ-1500S that came with my quilting frame (which I bought on Ebay several months ago).
I rethreaded the machine with new, high quality (Gutermann) cotton thread, checked the bobbin and made sure it was inserted correctly.
I sewed a straight seam. The top tension was a bit loose so I tightened it slightly.
I sewed another straight seam through two layers of homepsun - beautiful!
I sewed another straight seam through a quilt sandwich - even more beautiful!
I changed feet. I dropped the feed dogs. I attempted some free motion quilting. The problems with stitch length and wonkiness are due to my inexperience - the machine appeared to be working just fine.
I carried the machine out to the quilting frame in the garage.
I moved the needle just inside the two selvages of my two layers of calico (muslin) on which we have had successful quilting practice before.
I hand-lowered the needle to pull the bobbin thread through to the top.
I sewed three or four stitches in place to anchor the threads and I tried to sew a straight line.
Why then, having worked brilliantly in the kitchen, did I get this on the bottom of the layers before the top thread snapped?

Please, please help me!!