You’re sewing along, probably humming a tune to yourself when WHAM! Your needle jumps. Or the fabric moves in a wrong direction. Or your bobbin tries to pull the fabric into its dark and linty lair.
If you don’t have to pull your project off the machine to rectify the problem right away, don’t lose your stride to fix it now. Keep sewing that very, very long hem. Just first put a pin in the fabric next to your booboo so you can go back to it when you’re finished with your current task.