I was making squares in a square blocks today...chain piecing one corner. Got down to the last round, went to pick up the corner of the next block, there wasn't one on the stack. Looked at the block I was supposed to sew in to and saw what looked like I had sewen the last piece wrong sides together. what it was I had picked up two pieces and sewed as one. Had to do some snipping and resewing.
Someone mentioned using an industrial machine and got the cat tail in the bobbin winder. (which I thought was so funny!) Anyway, I used to use an industrial machine with the bobbin winder that would wind bobbins while you sewed. Sometimes I would cut the thread and not start winding a new bobbin. The thread would drop down and get caught around the pulley of the motor and I wouldn't realize it until I needed to wind more bobbins. If you haven't ever tried to get thread off the shaft of the pulley to the motor, good for you. Gets kind of hard once it starts packing in to the motor.