Venice's St Mark's Square was under water yesterday after a newly installed system of mobile artificial dams failed to activate.
Residents - long accustomed to perennial "acqua alta" or high water events - pulled on their rubber boots once more to deal with flooding that reached a high of 1.37 metres (4.5 feet) above sea level in the afternoon.
The waters drowned St Mark's Square - the Renaissance city's lowest area at about one metre above sea level - and invaded the famous basilica as many shopkeepers blocked their entrances with wood panels to keep the water out.
A massive flood defence system called MOSE aimed at protecting Venice's lagoon during high tide was finally installed in October.
The network of water-filled caissons is designed to be raised within 30 minutes to create a barrier capable of resisting a water rise of three metres above normal.
But on Tuesday the system failed to swing into action because the forecast erroneously predicted a rise of only 1.2 metres above sea level.
"To activate MOSE a bigger forecast is necessary," Venice Mayor Luigi Brugnaro told the Italian news agency Agi.
"We will have to review the rules of the command post."
The water reached a peak of 1.87 metres (six feet) above sea level on 12 November 2019, one of the highest recorded. Dozens of churches with UNESCO World Heritage status were damaged.
The MOSE infrastructure project began in 2003 but was plagued by cost overruns, corruption scandals and delays.
The project has cost about €7 billion, versus an original estimate of €2 billion.