You can forget to bring your ride pass with you, or forget to purchase a ticket or accidentally purchase the wrong ticket. Or e.g. with Caltrain you can forget

You can forget to bring your ride pass with you, or forget to purchase a ticket or accidentally purchase the wrong ticket. Or e.g. with Caltrain you can forget to swipe your clipper at the boarding station. Jumping a turnstile isn't merely oversight; it's intentionally trying to avoid paying the fare.