It's never good when a famous painting gets damaged! There are different types of damage that can affect paintings, some of them accidental, some intentional – such as the vandalism which affected The Rokeby Venus in 1914.
A lot of the work done here at National Gallery is actually repairing the damage to paintings. There is a painting that was very difficult for the gallery to fix!

A Garden to the Guildhall: a free Time Hunters history walk around the City of London
44:19

A Bridge to the Barbican: a free Time Hunters history walk around the City of London
50:06

The Great Fire Trail: a free Time Hunters history walk around the City of London
1:02:38