Like the time I forgot to move my biggest baddest unit in a 40k game... and only noticed when I tried to use it to fire on a target that was outside its fire arc. Or the countless times our D&D group decided the GM wouldn't possibly pit us against an enemy we couldn't defeat... ignoring the subtle hint to turn back.
Recognising significant moments in a game is almost as hard as it is in real life. The turning point in a war/campaign/battle is rarely obvious until after the event. Hindsight, as they say, is a wonderful thing.