A German museum is counting the cost after a cleaning woman mistook a valuable sculpture for an unsightly mess - and damaged it beyond repair.
The Martin Kippenberger installation entitled When It Starts Dripping From the Ceiling was on display at the Ostwall Museum in Dortmund.
The late modern master had created a tower of wooden slats under which a rubber trough was placed with a thin beige layer of paint representing dried rain water.
Taking it for a stain, the cleaner scrubbed the surface until it gleamed.
"It is now impossible to return it to its original state," a city spokesman said.
She said the work, valued by insurers at 800,000 euros (£690,000), had been loaned to the museum by a private collector.
Cleaning crews had orders to keep 20cm (8ins) away from artworks but it was unclear if the woman had been informed of this by the contractor that employed her.
It is not the first time works of art have suffered at the hands of over-zealous cleaners.
In 1986, a 'grease stain' by Joseph Beuys valued at around £346,000 was mopped away at the Academy of Fine Arts in Dusseldorf, western Germany.