哪壺不開提哪壺

Etymology
Literally: whichever pot is not boiling, pickup that pot
 * If a pot of water is not boiling, it does not make any (high-pitched) sound. This means figuratively that the pot of water is defective. If one deliberately picks up such a pot, then it figuratively means that one is deliberately publicizing somebody else's weaknesses or secrets.

Idiom

 * 1) to air somebody's dirty laundry
 * 2) to blurt out somebody's weakness
 * 3) to let somebody's skeletons out of the closet