It is possible that the shut-off valve was blocked by the cumulated white stuff. As a result, water supply can not be stopped even when water level is high. : H; g$ p+ B5 R3 l& s4 W: o B$ U: T& F2 n) T* J) ?
Solution: turn off water supply and take the humidifier apart and clean it will stop the leaking. & m' r4 B$ K8 ~' T( f- d : I/ ~: Z, W. y8 n) O1 W0 h8 y% j2 S n* ^* l+ j) \ A' |$ C6 O
Otherwise, find a professional or change a new set.