Reina Sofia museum in Madrid "lost" a sculpture by American artist Richard Serra. I'm sure in itself that's not unusual, but the lost sculpture is made from 38-ton of steel!
This year the museum admitted losing track of the work, Equal-Parallel/Guernica-Bengasi, for which the culture ministry paid about £150,000. ...
A police investigation concluded that theft was implausible. The piece, which had taken five cranes to move, was thought to be too awkward to handle, and it was worth almost nothing.
It was thought the artwork had not been moved but was incorporated into the foundations or grounds of the ministry.