Tom Riddle, when he had finished school, asked Dippet if he could stay on as a teacher, but Dippet told him that at eighteen he was too young (HBP20).
During Dippet’s tenure as Head, an infamous performance of a pantomime of The Fountain of Fair Fortune nearly burned down the Great Hall (TBB/FFF).
Rita Skeeter wrote a tell-all biography of Dippet entitled Armando Dippet: Master or Moron? (DH13).
"dip" = U.S. slang for someone who is rather inept and silly.