Oldest flagDanish (Dannebrog) - the most ancient, documented history of which dates back to the 14th century.
Military flagPhilippines is the only country in the world whose flag is changing during the war. When the country is at war, the flag is turned over so that the red bar is over the blue (and usually there is blue over the red).
VersatileFlag Paraguay - this is the only flag of the state in the world, which has different emblems from the front and back sides.
On the front side there is a coat of arms, with no back.
Custom shapeFlag Nepal is the only non-rectangular flag in the world. Switzerland and Vatican - square flags.
Completely the same flagsThere are many such flags, but there are and quite the same. Monaco and Indonesia. When Indonesia changed its flag, it turned out that it is exactly the same as that of Monaco. Monaco is not pleased and she sued. But it turned out that Indonesia already had such a flag and it just returned to its historical roots.
So both countries have the legal right to use their flag. But none of these stubborn people refused to yield Liechtenstein at the 1936 Olympics saw the flag Haiti, exactly like their flag. But they, unlike the ones described above, did not become crusty, and the crown was added to the flag. Romania and Chad are almost identical, even though Chad's blue tone is slightly darker. But it is noticeable only by placing them side by side. Ireland and Côte d'Ivoire are also quite the same, only in different order - from left to right and vice versa. But the wind blows in different directions, so the flags are just the same.
Nevertheless, they believe that their flags are different.
Flags - SymbolsWhite flag of truce. The black flag, and black color as such, have been associated with anarchy since the 1880s.
Unofficial flagsThere are Christian flags, Esperanto. Gas stations have their own flag, why not come up with their own personal? :)
The flag of the planet is more interesting. The flag of Mars is a tricolor representing the planet. Although it has no legal force, the flag was approved by the Martian community and the planetary community. The flag is intended to depict the "future history" of Mars.