(Danganronpa 1 spoilers and Chapter 3 (in DR2) spoilers)
This was kinda tricky for me in the first game where you have to nail the biker dude on him knowing the color of Chihiro's gym uniform (or something like that). As for this case, nothing seems particularly odd about the first three things she brings up (the hemp bag, hospital gown, and slippers are all pretty plain-to-see). Probably something with the differing body types/camera angle/candle lighting.
I suppose my best guess at this point is in her knowledge of how similar the conference room is to the stage, where the black curtain would probably be most relevant.
Extra Hint:
The previous one in the first game was more about what they said earlier that gave it away. This one is about what she's saying in the nonstop debate that she shouldn't know.
Bigger hint:
Its a subject that she has no reason to use as an excuse unless she was part of making the video. Hajime didn't talk about it at all and it wasn't brought up at all before. I believe you need to take the mistake comment and shoot it at it.
Answer Hint:
She shouldn't know anything about the angle the video was filmed at. Her trying to use it as evidence for her defense is damning.