Being an owner of one of the world's worst passports (Henley & Partners ranked Sri Lanka the 6th worst), I always have to worry about visa.
Other than the United States, what countries require a visa for connecting flight? In all countries I have visited, one can stay in the airport terminal for a short period of time to board to a second flight. I understand this is not the case for all countries/administrative areas.
If I have a layover in some Airport, what countries require a visa even for a short layover without leaving the airport? I believe the situation would be different and a visa is mandatory for cruise ships, etc.