Renting in one country and driving into another.
Not all rental agreements permit crossing an international border, and those that do often require advance notice, an additional fee, or specific documentation like a green card for insurance purposes. Confirming this directly with the rental company before booking, not after, avoids a problem at the border.
Some countries also have specific requirements for foreign-registered rental vehicles, like additional stickers or documents, that are worth researching ahead of a cross-border drive.