My daughter did......and she did it while she was "helping" me push the cart. There was something on the floor in the aisle and she didn't see it, tripped over it and skinned her knee. Accidents happen, no matter how close to you the child is, or how vigilant you are as a parent. That has been my entire point all along........you just refuse to acknowlege it.
If you had taught your daughter to look out for bumps in the aisle, she would not have fallen. You failed to do so and so she fell and skinned her knees. Of course, if she were living in the 50s, none of that would have happened. It just didn't. ;)