Suddenly the sound coming from the area of the needle wasn't the same. Experience has taught me that's not a good thing - so I stopped and looked closely. Yup. The stitches on top looked "wrong", and on inspecting the back, I could see a birds-nest of long threads dangling loosely. Fortunately I stopped immediately when I heard the sound, so there was only about an inch or so to pick out.
I pulled the bobbin case out, expecting that the thread had slipped out of the little metal clip on the side (which will cause the birds-nest problem), but no... the bobbin looked fine. Hm... So I popped it back in, did a little test quilting on a scrap piece I use for this purpose - and well, the stitches looked fine. So I went back into the quilt and continued working.
Half an hour later, as I was nearing the end of the quilting, the machine suddenly jammed and the screen on the longarm showed the error message "Needle Sensor Failure on Walk". I pulled out my user manual and checked the troubleshooting section - but there isn't any mention of this error message. Looking at the stitching, I could see that the last 2 inches or so were "birds-nest" stitches again.
Again I checked the bobbin case - nothing obviously wrong there. I turned the hand wheel on the back of the machine to manually raise and lower the needle - not seeing any problem there. So I reloaded the bobbin case - grabbed my test scrap and tried stitching... the machine was working again.
I finished the quilting without further incident but became worried that something really bad was wrong.
A search on Google gave me some clues. Apparently that error can be caused by a number of things, including:
- bent needle
- foreign objects stuck in or around the bobbin case (such as a broken needle tip)
- loose, stray threads around the bobbin case
That seems to have taken care of the problem. Got my fingers crossed.