Flaps blown by prop wash - I can recall the Breguet 940, 941 and 941S, the Ryan VZ-3 Vertiplane and the VZ-5 Fairchild Fledgling. Then the jet versions, the two Coanda machines, Boeing C-14 and Antonov An-72/74. Then there was the McDonnell Douglas YC-15 where the jet efflux blew over the flaps, leading eventually to the C-17.
Here is the French Breguet 941 with flaps fully extended into the slipstream and flying in a markedly nose-down attitude.