Polish soldiers mistakenly crossed the country's border with Czech Republic in late May before setting up there, the Czech foreign ministry told CNN.
The soldiers, who had been guarding parts of the closed Polish-Czech border during the coronavirus pandemic, then started turning away Czech citizens who were attempting to visit a church in their own country.
The snafu led the Czech embassy in Warsaw to take "immediate action" and notify its Polish counterpart, the Czech government told CNN, adding that Poland has still not formally explained why it mistakenly annexed its neighbor.
"Our Polish counterparts unofficially assured us that this incident was merely a misunderstanding caused by the Polish military with no hostile intention, however, we are still expecting a formal statement," a Foreign Ministry spokesperson told CNN.
"The Polish soldiers are no longer present and our citizens can again visit the site freely," the spokesperson added.
"We are still waiting for the formal Polish statement," they repeated when asked how long Polish troops were in the country.
Poland's Ministry of Defense also acknowledged the brief occupation on Friday.
"The placement of the border post was a result of misunderstanding, not a deliberate act. It was corrected immediately and the case was resolved -- also by the Czech side," it told CNN in a statement.
The incident occurred near Pielgrzymow, a small border village in southern Poland that sits opposite a sparsely populated stretch of Czech countryside. A quiet road there serves as the boundary point between the two nations.
"Soldiers of the Polish Army support the Border Guard in protecting the state border after its closure due to the coronavirus pandemic," the Polish Defense Ministry explained. "The operation is led by Armed Forces Operational Command, which is in direct contact with the Border Guard."
The Polish government did not confirm how long its soldiers were present in the Czech Republic.
Borders between European Union countries are often barely visible as citizens enjoy freedom of movement across the bloc.
But the coronavirus pandemic has complicated that longstanding arrangement, with nations shutting off entry to foreigners to control the spread of the virus. Poland has blocked people from entering the country since March.
Poland was involved in a handful of more serious border conflicts with the former Czechoslovakia during the 20th century. The two countries fought a seven-day war over territory in the Silesia region in 1919, and Poland annexed a region around the city of Bohumin in 1938.