Revealing mistake: When Katniss is blown back from the explosion, her stunt person is visible. This can be proven because the stuntperson's skin tone is different and her hair is not in a braid. It is in a ponytail. Wires can also be seen pulling her to give the illusion that she was blown back from the explosion. (01:33:00)
Revealing mistake: Katniss shoots and kills someone who throws a spear at Rue. When Rue says "you have to win" we see a shot of the dead bloke with the arrow sticking out of him. In this shot, if you look carefully you can see a square outline under his shirt that the arrow is stuck in. (01:36:20)
Answer: I watched this scene multiple times on YouTube. There is definitely something moving that probably is not supposed to be seen. I'd agree with your assessment that it looks like a hand. Most likely a film crew member is crouched on the floor performing some technical function during the filming. Just what it is impossible to tell. You should submit it as a "movie mistake."
raywest ★