Failure to provide Advance Passenger Information (API)—which includes your passport details, date of birth, and nationality—can lead to significant travel disruptions. In 2026, most airlines and international transport operators (like Eurostar) require this data well before departure to comply with government security mandates. If you forget to provide it, you will likely be blocked from checking in online or via mobile apps. At the airport, you will be pulled aside at the check-in counter to manually enter the data, which can cause you to miss your flight if lines are long. In some strict jurisdictions, such as the U.S. or the UK, passengers who have not cleared the "no-fly" or security screening triggered by API may be denied boarding entirely. Furthermore, if the airline allows you to board but fails to transmit your data, they can face massive fines, and you may face a much longer and more invasive interrogation by customs and border officials upon arrival at your destination.