(2001) It is often said that modern art is rubbish, but never did it ring as true as when an art gallery cleaner binned a work by Damien Hirst because he thought the installation was exactly that – leftover rubbish.

Emmanuel Asare thought the piles of full ashtrays, half-filled coffee cups, empty beer bottles and newspapers strewn across the gallery were the remnants of a party in the west London gallery.

Although that is what it was, this rubbish had been arranged by Hirst into an impromptu installation, which increased its value by thousands.

Hirst had arrived at the launch party of a new exhibition of his work, Painting-By-Numbers, on Tuesday night at the Eyestorm gallery. When asked how he would like his prints to be displayed in the windows, the artist, famed for his formaldehyde animals, decided to create a new work.

The next morning when Mr Asare arrived for work, he decided to clean up the mess straight away by putting it all in bin bags. Mr Asare said: “As soon as I clapped eyes on it I sighed because there was so much mess.

“I didn’t think for a second that it was a work of art – it didn’t look much like art to me. So I cleared it all into binbags and dumped it.”

Staff were dispatched to find the binbags in the rubbish, and salvaged the various objects, which they used to reconstruct the installation from photographs taken earlier.

(2011) An overzealous cleaner in Germany has ruined a piece of modern art worth £690,000 after mistaking it for an eyesore that needed a good scrub.

The sculpture by the German artist Martin Kippenberger, widely regarded as one of the most talented artists of his generation until his death in 1997, had been on loan to the Ostwall Museum in Dortmund when it fell prey to the cleaner’s scouring pad.

The work, called When It Starts Dripping From the Ceiling (Wenn’s anfängt durch die Decke zu tropfen), comprised a rubber trough placed underneath a rickety wooden tower made from slats. Inside the trough, Kippenberger had spread a layer of paint representing dried rainwater. He thought it was art: the cleaner saw it as a challenge, and set about making the bucket look like new.

A spokeswoman for the museum told German media that the female cleaner “removed the patina from the four walls of the trough”.

“It is now impossible to return it to its original state,” she said, adding that it had been on loan to the museum from a private collector and was valued by insurers at €800,000 (£690,000).