Some RAF ones:
Decision not to produce the Spitfire Mk III, which would of significantly ameloeriated the performance advantage the LW possesed during the 'Focke Wulf summer'. Had the Mk III been produced it would of been significantly faster, longer ranged and better armed then the Mk V.
Decision to abandon the concept of a support bomber/stroke fighter, and hence not to produce the Hawker Henley as a bomber and not procure the Gloster F.9/37 for development . The Henley was some 60 mph faster, more manouverable and more survivable than Faiery Battle, and would of worked much better as a close support bomber. The Gloster F.9/37 would of given the RAF an alternative to the Whirlwind as a twin engine, single seat fighter, particularly good below 15,000 feet. The F.9 would of been a very capable strike fighter until the Mossie came along.
Decision to allow Rolls Royce to kill development of the Peregrine engine, effectively ending the Whirlwind programme at the Mk I with just 144 airframes produced, denying the RAF an effective single seat escort fighter until 1944.
FAA policy of a dedicated crew of two for fighters, essentially neutering the performance of the Faiery Fulmary and Firefly, making them some 1000 lbs overweight.