In Prague's historic Old Town Square, you will find an intricate 600-year-old piece of machinery called the Orloj.
The colorful medieval clock is one of the most famous sights in Prague, adorning every tourist guide and attracting large crowds who gather to watch it work.
It's something of a symbol of national pride, says Prague Radio International's Thomas McEnchroe.
"You have all these sort of little puppets that move and dance. So it is a very popular attraction for tourists who come here," he said.
The Orloj has been repaired several times since it was installed in 1410. The last major renovation cost the city $2.6 million and was finished in 2018. That's when art restorer Stanislav Jirčík was commissioned to paint a replica of the clock's famous rotating calendar plate.
"Imagine it as a big circular picture on which different months are artistically depicted as images," McEnchroe said.
But four years after Jirčík finished it, a member of a local preservation group noticed something was off.
"He just noticed so many mistakes. The images are very different to the original," McEnchroe said.
There weren't major differences, just lots of little things. Like a dog that was black in the original is now brown in Jirčík's version. Or a figure is painted older or with different hair.
The more you look, the more changes you find.
Some people think Jirčík was playing a prank on the city. Others think he painted his own friends into the piece. Jirčík has rebuffed the criticism and said he was inspired by the original artist.
"The Czech Republic is in the news, and it's a bit embarrassing on one hand," McEnchroe said. "And on the other hand, it's just a sort of strange joke that nobody noticed this for four years."
For now, the new painting is still on display in the Old Town Square, but investigators from the Ministry of Culture are on the case.
And surely, they are working around the clock to solve it.