This tip is written for Bangkok, but really it could be applied to any country.
Many countries require a departure tax, but the tax is built into the price of the plane ticket, so you're not even aware of it. However, some countries require a departure tax that is NOT built into the plane ticket price.
Thailand is one such country. You basically cannot leave the country, let alone board the plane, if you do not pay the departure tax. The booth to pay the departure tax is separate from the check-in booth, right before security. Be sure to have the right amount of Baht with you, or you will be sent to the ATM or to Currency Exchange.
