In the scene near the end where Sarah is auditioning, performing her contemporary dance piece, at times she is wearing socks or some kind of covering on her feet, and at other times she is barefoot. This is only a minor slip-up, but I notice it every time, and I am surprised no one had caught it before me.
In one of the dance scenes when Derek and Sarah are dancing, the part when everybody is watching them, Derek does a split, but if you pay close attention you will recognize its not Derek because you see a light skinned person and Derek is dark.
I thought that too at first but when I watched it carefully, she's wearing ballet shoes. From the camera angle they blend in, and look like she's not wearing them.
Well, no there was no disappearing socks!!! Sorry if you knew anything about dance you would know that there are convertible tights which are tights that have a hole in the bottom of them so you can role them up so she probably did that while she was taking off her point shoes.