Private Swap - Czech Republic

The national flag of the Czech Republic is the same as the flag of the former Czechoslovakia. Upon the dissolution of Czechoslovakia the Czech Republic kept the Czechoslovak flag while the Slovak Republic adopted its own flag. The first flag of Czechoslovakia was based on the arms, and was white over red. This was identical to the Flag of Poland, so a blue triangle was added at the hoist. The flag was banned by the Nazis in 1939, and a horizontal tricolor of white, red, and blue was enforced. The original flag was restored in 1945.

When the Czech and Slovak Republics separated in 1993, the Czechs adopted a flag identical to the former flag of the Czech and Slovak Federal Republic, in direct violation of the constitutional act that explicitly forbids former federal symbols to be used by the two successor states. (source) Thank you mailianne!