Girl forgets about her friend/roommate?

So, I don’t remember where or when I saw this, or the main plot, but early on, a female character ties up her friend/roommate (don’t remember which) for some sort of sexual fantasy. She is supposed to untie her shortly after, but something important comes up (again, don’t remember what), and she doesn’t remember until the very end. I’m pretty sure it wasn’t a movie, but it could have been pretty much anything else.

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