Czech Republic, 1990-1992
Czech readers will be forgiven if they don’t recognize this one. After the Velvet Revolution, the two federal components of Czechoslovakia adopted national symbols. Slovakia went with its tricolour and the Czech Republic used the traditional flag of Bohemia.
It didn’t really catch on for a few reasons: For one thing it looked just like the Polish flag and no one felt any real connection to it. But also the Czech population tended to be more in favour of federalism, so they mostly just flew the Czechoslovakian flag.
The plan for dissolution was that both new countries would use their new national symbols, but no one was really happy with the bicolour. The Czech Republic spent a few months searching in vain for a unique flag, but they finally gave up just two weeks before independence and officially adopted the old flag of Czechoslovakia.