Zuse Z3 Lost
The first working programmable, fully automatic digital computer — 2,600 relays, 22-bit floating point, Turing-complete in principle. Destroyed in 1943 bombing; the world didn't learn of it for years.
The Z3 ran in May 1941 in Zuse's Berlin workshop — 2,600 telephone relays, 22-bit floating point, programmed from punched film. Raúl Rojas proved in 1998 it was Turing-complete (with conditional loops added); at the time it calculated aircraft wing flutter for Henschel.
Destroyed in a December 1943 air raid. The world learned of it years after ENIAC's 1946 unveiling — which is why 'first computer' claims vary by what counts: electronic, stored-program, or merely working.