Can't stop watching this scene:
The acting was superb! When Margaery drops the "Holy Queen" act and starts fighting for survival, to the point where she said "Forget about the bloody gods!" and the High Sparrow's reaction was priceless, you could see that he was surprised to see this kind of response from her.
But yes, this scene shows you Margaery's fatal mistake, leaving things to the High Sparrow and in the end, he was too confident.