At home-coming, Mr. Boy offers Layla a drink. As he's handing it to her, Lash spills it on the teacher's shirt, but immediately after he spilled it, the glass is full again. Finally after changing his clothes, it's empty.
You're right on this. I noticed the mistake the first time I watched the movie, and that was in the theater. It's weird how a cup can be empty, then suddenly be half-full without anyone putting anything in it.