Overstaying your B-1 visa leads to immediate visa cancellation and accrual of "unlawful presence." If you overstay by more than 180 days, you face a three-year re-entry bar; over one year, a ten-year bar. You generally can't extend your visa or change your status post-overstay, and you risk deportation and complications in future U.S. visa applications. It's crucial to adhere to the I-94 form's exit date to avoid these serious consequences.