My bad habit is to stop and start with the needle up but the thread take up NOT all the way up. This can mean that the bobbin is ready to finish a loop and will make a cool knot down there or a piece of thread wrapped around the bobbin shaft to clean up. My habit is especially bad when I use my industrial machine. I've had to pull the shuttle off the shaft and dis-assemble it to get the trapped threads out. I did learn sewing machine repairs though. At least I know what causes it now.