What does the girl cutting her hand in Animatrix: Beyond symbolize?

There's a couple ways to interpret this scene.

The straightforward approach is to just assume she was cutting her hand with the can to see if maybe the 'power' they thought they had had to do with the can or with themselves, meaning maybe only organic matter (bodies, blood, etc) will work, but the blood still dripped to the ground.

The other more psychological approach is her realization that the place has no more 'magic' to it and the cutting is possibly an extreme way to feel something after having this area's special properties taken away from her.

Finally, it could just be dramatic effect and nothing more, though the stories of the Matrix have never been known to use imagery for something so basic, I believe.

If you watch the film carefully, you see that the can came from the "glitched" area and bounced out of quarantine as the Agents were resetting it. The can is the one object that survived their purge. When she cut her self, maybe she realized the truth, or had some form or power or aspect of the glitched area transferred to her.

Or maybe it was a straightforward sadness, loss-of-innocence scene. There's no way of knowing for sure. One of the most beautiful moments in film, regardless, and few will appreciate it.

I think it's consequence. The whole story is this "haunted house" that allows the kids to do whatever they want, free to explore and roam. It's childhood. The agents' truck is the looming, oncoming reality of adulthood. They dispose of the magical playground, and the main character feels hurt, now outside of the consequence-free environment they came from.

I experienced it as a metaphor of blood - life, the latter in the sense of experiencing "real" life. The "glitch" experienced with the kids opens a door for the character to see that there is more to this life than the "bloodless" one she was living before, a door that before was invisible. In a way, her act of dropping the blood in the floor while slowly breathing resembles saying "I am here, I am real, I can feel". For me, somehow this act is connected with the glitch, which showed her this new door of freedom, deepness, or awareness. Blood becomes then a metaphor for life.