Stalingrad does seem like a dumb decision no matter how it is sliced, as was allowing the RAF to rebuild, or allowing the Enigma machines to be used lazily so that the Brits could crack the otherwise-uncrackable ciphers. Then again, even Patton, who stated that permanent fortifications were ineffective, "monuments to man's stupidity", had a moment where he became bogged down attacking an established position instead of going around it and cutting it off. There's also some report that Hitler never truly declared war on the USA. History contends that Hitler did declare war on the USA, which would have been his biggest mistake if true, particularly since Japan was being an anti-bro and refusing to attack the USSR.