When the prince goes to put the glass slipper on Cinderella's LEFT foot, the step-mother causes it it break. Cinderella then comes back w/ the other slipper and puts it on her LEFT foot.
Actually, when the first stepsister (the red-haired one) is trying on the shoe and it doesn't fit, the stepmother says, "...Are you sure it's on the right foot?" implying that there ARE right and left shoes.
Actually, it was the servant who was trying the glass slipper on the girls, and the stepmother uses her cane to trip the guy carrying it, so yes, she causes the slippers to break.
Just a minor note, it wasn't the prince who were fitting slipper on every maiden's foot, it was the king's assistant with a servant. And the slipper they brought never get to be fitted on cinderella's foot. It broke just before the guy got to put it on cinderella's foot.
But to answer your question, this one is NOT a slipup. The 'lost slipper' was a left, when he fitted on Drizz
ella's foot, her thumb toe clearly shown nearest to the camera, thus left foot.
And when Cinderella produced the other slipper and put it on, her pinky toe was shown nearest to the camera, thus right foot. (They both sat facing right when trying on shoe)