I learned the hard way. If your machine starts sounding different, investigate. Do not just keep sewing.
I was sewing along, doing my feathers, when I heard the machine change its tone, but the stitching was fine, so I kept going. Then I felt a "bubble" under the fabric. I tried to smooth it but it wouldn't. So I looked underneath. This is what I saw!
Lesson 1- investigate sooner than later.
Lesson 2- keep my sewing area cleaner so I don't pick up hitchhikers!
picked up a piece of binding strip on underside of quilt