Not high level but I am a tourney player.
Oh yes, all the time: Unfamiliarity with a matchup or they play so radically different and random from how a character "should" be played that it throws me, dropping combos and punishes for various reasons, etc..... I lost to a Cyborg that, when they were close enough, constantly did jump-in 2, sweep and, if they got the knockdown, go for a (fake) crossup into some basic combo. Next person to try this I beat but the first guy it was just like "WTF is this?".