Leaving aside luck, which fortunately or unfortunately can play a big part in the outcome, I see pitfalls on both end of the spectrum.
One, I think of as a "GM with a j frame," where someone overestimates the value of their technical ability at a game, and applies that to a lethal encounter. Two, is the timmie that believes mindset is a substitute for developed technical skills, and will somehow allow them to rise to the occasion. I like that Barrett Tillman quote, which more or less is "that in an emergency, you will default to your level of training."