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!

Oh man! That really stinks! I find that I am getting very well acquainted with my seam ripper these days.

ouch!!

I would like to offer my sympathy. You poor thing.

hitchhikers...that's funny!!!

been there, done that!! Never any fun

Nope, no fun at all to pick up hitchhikers, lol!

Sorry!!!

Sorry that happened. Thank you for reminding me. A stitch in time saves nine.

Bummer!

I have something similar happen, when the backing folds over the quilt and I sew it over. Now I don't make my backing so much bigger, so I can keep track of it.