The benefits of hindsight: remove defensive armament from RAF heavy bombers in 1943. Subsequent news headlines "our boys have no defence over Germany".
As per several (many) previous posts on Pprune, the Mossie (or similar) should have been the bomber of choice over Germany (with hindsight), but no-one in a position of authority at the time would have instructed Avro et al to build twin engined bombers with no defensive weapons instead of four engined aircraft bristling with machine guns, in case the decision was wrong.
Perhaps the greatest blunder is - as always - building aeroplanes designed to win the previous conflict.