l'acqua cheta rovina i ponti
(calm water ruins the bridges) •
Referring to the erosive power of water, it's used to describe people that come across as quiet, but they do their work without being noticed (the work having a negative connotation).
"Hai sentito che ha combinato Luca? É riuscito a fregare tutti! Chi lo avrebbe mai detto!"
"Eh, l'acqua cheta rovina i ponti!"
"Have you heard about Luca? He screwed everybody! Who could have possibly told!"
"Eh, calm water ruins the bridges!"