Lol it happened with the nazis. The diaries of their leaders at the start of the invasion of the ussr were like "this is the biggest movilization ever! Our victory is inevitable and glorious", 3 weeks later goebbels was like "our intel was completely wrong".
apparently the main logistics guys actually calculated how barbarossa would go but then their chief buried the report because everyone around him kept going on about the triumph of will and shit
and then the invasion went exactly as the logistics guys predicted it would.
also, gotta love the overall nazi logic of knowing that if they dont win by christmas, they have no path to victory and will lose, and deciding "we'll just win by christmas then! contingency? what's that? we have aryan will!"
It wasn't his generals, Mein Kampf is just hundreds of pages of incoherent racism/anti-semitism + declaring the destiny of Germany to be conquering the weak commie lands and plundering/enslaving them. I believe the paraphrased translation of a quote was that the USSR was a rotten door that would crumble when kicked in. He made the exact same error.
I've got this special interest with regard to the amazingly-wide gulf between the perception of how effective the Amerikan military is, and how incompetent they actually are. tl;dr, practically all of our wargame simulations are heavily scripted-- when they don't script how OPFOR "is supposed to act"(direct quote from one of the overseers of MC'02), OPFORs rungs lower on the tech ladder absolutely TPK our shit with low-tech methods. Millennium Challenge '02 is a prime example of the kind of fuckery the Joint Chiefs get up to fudging how effective our troops are.
I imagine they have moments of clarity/more realistic factions, but that's constantly undermined by "USA #1" rah rah bullshit and "those people can't possibly beat us" racism.